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Full text of "Field Artillery Radio Repairman's Handbook"

Department of Communication 
April 1944 



FIELD ARTILLERY 
RADIO REPAIRMAN'S HANDBOOK 



Short title: FARRH 




LIBRARY- , 
FMd Artillery Scud. - 
Fort KM, Oklahoma 

^27 ? 77 

Published April 194* at the Field Artillery School and dietrlbuted with the approval of the 
Commanding General. Army Ground Force., letter 30O.SF/6 (14 April 1944) GNRQT-2/78390. 



FIELD ARTILLERY SCHOOL 
Fort Sill, Oklahoma 



ipMifcno 



lo 



RESTRICTED 

Department of Communication 
April 1944 



FIELD ARTILLERY 
RADIO REPAIRMAN'S HANDBOOK 



Short title: FARM 



t^.S. 7,/T.S. 




Published April 1944 at the Field Artillery School and distributed with the approval of the 
Commanding General, Army Ground Forces, letter 300.5F/6 (14 April 1944) GNRQT-2/ 78390. 



FIELD ARTILLERY SCHOOL 
Fort Sill, Oklahoma 

RESTRICTED 



TABLE OF CONTENTS 

Paragraphs Page 

Chapter 1. General 1- 2 1 

Chapter 2. Test equipment. 

Section I. General 1 . 3-5 3 

II. Current measuring instruments 6-15 4 

III. Voltage measuring instruments __ 16-25 6 

IV. Power measuring instruments 26-27 10 

V. Continuity indicators _■_ 28-32 10 

VI. Signal sources ._ __ 33-37 12 

VII. Signal tracer ._ 38-41 13 

Chapter 3. Testing radio parts and assemblies. 

Section I. Fundamental parts 42-48 15 

II. Special units 49-68 21 

Chapter 4. Testing circuits. 

Section I. Fundamental vacuum circuits 69-74 27 

II. Special vacuum tube circuits 75-82 28 

III. Power supply circuits .:.... 83-90 32 

Chapter 5. Testing stages. 

Section I. Receiver stages . 91-102 39 

II. Transmitter stages 103-110 49 

III. Alignment procedure 111-112 56 

Chapter 6. Testing systems. 

Section I. Voltage and resistance measure- 
ments - 113-126 59 

II. Signal tracing 127-130 65 

III. Other operational tests -13 1-133 71 

Chapter 7. Testing transmitter systems 134-141 75 

Chapter 8. Replacement, repair, and 

maintenance . 142-159 79 

Chapter 9. Service methods. 

Section I. General , . 160-161 85 

II. Receiver . 162-184 . 86 

III. Transmitter .. _. 185-188 ,95 

Chapter 10. Vehicular interference. 

Section I. Causes and location of trouble 189-193 98 

II. Noise suppression 194-199 100 



Paragraphs Page 

Chapter 11. Radio sets SCR-609 and SCR-610. 

Section I. General 200 103 

II. Presetting channels 201-203 103 

III. Alignment and neutralization „._204-208 107 

IV. Servicing . - 209-212 109 

Chapter 12. Radio sets SCR-608 and SCR-628. 

Section I. General 213 116 

II. Presetting channels . 214-217 116 

III. Receiver alignment 218-222 119 

IV. Transmitter alignment 223-229 121 

V. Servicing 230-234 123 

Chapter 13. Radio set SCR-284. 

Section I. General — 235 132 

II. Alignment and neutralization —-236-241 132 

III. Servicing - 242-245 134 

Appendix I. Abbreviations _ — 14 1 

II. Parts color coding 142 



Index 



145 



CHAPTER 1 
GENERAL 

1. PURPOSE. The purpose of this manual is to provide field 
artillery personnel with basic principles of maintenance and in- 
spection of radio sets and to provide field artillery radio repair- 
men with fundamental techniques and procedures necessary to 
accomplish first and second echelon maintenance. A thorough 
knowledge of the principles of electricity and radio contained in 
TM 1-455, Electrical Fundamentals, and TM 11-455, _ Radio 
Fundamentals, is required. Ability to use the techniques con- 
tained in TM 11-453, Shop Work, is also necessary. 

2. ECHELONS OF MAINTENANCE. 

a. General. The basic policy to be followed in all maintenance 
and repair of communication equipment is prescribed in TM 38- 
250, Basic Maintenance Manual. 

b. First echelon maintenance. The first echelon is preventive 
maintenance performed by the radio operator. This phase of 
maintenance is continuous regardless of time or location. Mini-' 
mum training objectives include the following: 

(1) Proper operation of equipment as specified in the ap- 
propriate technical manual. 

(2) Proper care of equipment, including protection from 
weather and rough handling. 

(3) Cleaning all external surfaces and removing dust and 
foreign matter (such as battery corrosion and excess grease) 
from internal parts. 

(4) Preservation of exposed leather, web equipment, and 
external surfaces by timely and proper application of approved 
paints and preservatives. 

(5) Inspection of equipment and installations to determine 
their ability to operate satisfactorily over the required distance. 

(6) Minor adjustments necessary to transmit and receive 
on various frequencies. In the case of crystal-controlled radio 
sets, this includes changing crystals and retuning sets. 

(7) Replacement of parts where replacement does not in- 
volve fixed electrical or mechanical connections. This includes 



such items as antennas, vacuum tubes, microphones, headphones, 
batteries, power supply units, lead-in wires, and connecting cords 
and cables. 

(8) Reporting all defects and troubles to the appropriate 

commander. 

c Second echelon maintenance. The second echelon mainte- 
nance is additional maintenance performed by the radio repair- 
man and supplements that of the first echelon, it is limited large- 
ly by the tools, parts, and time available, and by the skill required 
to accomplish a particular job successfully. In no case should, 
adjustments or repair of a radio set be attempted in the second 
echelon unless successful completion^ is a certainty. Minimum 
training objectives include the following: 

(1) Periodic technical inspection of all radio sets. 

(2) Additional preventive maintenance. 

(3) Adjustment of any controls provided on the set. This 
includes tuning; alignment of RF, IF, discriminator, and BFO 
stages; neutralization; and adjustment of couplings. 

(4) Location of electrical or mechanical defects. 

(5) Determining if the necessary repairs can be made with 
the tools, test equipment, parts, and skill available. 

(6) Repair of electrical or mechanical defects within the 
limits indicated in (5) above. 

(7) Tagging sets which cannot be repaired in the unit, and 
indicating the defect for the convenience of the next higher eche- 
lon of maintenance. 

(8) Making a final operating check to determine whether 
the set is operating satisfactorily and if it will communicate over 

its rated range. „,.,., 

(9) Reporting to the communication officer all sets which 
cannot be repaired in the unit. 



CHAPTER 2 
TEST EQUIPMENT 

Paragraphs 

Section I. General 3- 5 

II. Current measuring instruments : 6-15 

III. Voltage measuring instruments 16-25 

IV. Power measuring instruments 26-27 

V. Continuity indicators 28-32 

VI. Signal sources 33-37 

VII. Signal tracer 38-41 

SECTION I 
GENERAL 

3. GENERAL. Test equipment of various types is used to con- 
trol the operation, channel presetting, alignment, and testing of 
all field artillery radio sets. A thorough knowledge of the use 
of available test equipment is indispensable to repairmen. Ap- 
propriate technical manuals must be consulted to determine pre- 
cise methods for using each item of equipment. The theory of 
operation of various types of meters is included in TM 1-455, 
Electrical Fundamentals. 

4. CLASSIFICATION. OF TEST EQUIPMENT. Test equipment 
may be classified according to its principal use as current 
measuring instruments, voltage measuring instruments, power- 
measuring instruments, continuity indicators, signal sources, 
signal tracers, and tube testers. 'All classes of test equipment 
except tube testers are considered in this chapter. Tube testers 
are discussed in chapter 3. 

5. REPAIR AND MAINTENANCE. The extent of repairs which 
may be accomplished on test equipment by the repairman will 
be limited by the nature of the trouble, the individual repair- 
man's skill and ingenuity, and by the equipment available. 



SECTION II 
CURRENT MEASURING INSTRUMENTS 

6. METER MOVEMENT. Most current measuring meters use 
the same basic movement. A small coil, suspended on jeweled 
bearings and free to rotate against a retarding force of two spiral 
springs, is mounted between the poles of a permanent magnet. 
An electric current passing through the coil sets up a magnetic 
field around it which reacts with the permanent magnetic field 
and causes the coil, which has a pointer attached, to rotate on 
its bearings. Deflection of the pointer from zero will indicate the 
amount of current flowing. Each meter is designed to allow full 
scale deflection of the pointer when a definite current is flowing 
(commonly 1 milliampere, 400 microamperes, or 250 micro- 
amperes) . 

7. CONNECTIONS. When measuring current flow, the meter 
must be connected in series with the circuit being tested, so that 
all or a known proportion of the current flows through the in- 
strument. If accurate measurements are required, the instru- 
ment resistance must be low in comparison to that of the circuit 
being tested. 

8. DETERMINATION OF AMMETER RESISTANCE. To deter- 
mine the resistance of an ammeter, connect a battery, ammeter, 
and variable resistor in series as indicated in figure 1. The vari- 
able resistor must be sufficiently large to limit the current 
through the meter to a safe value, and must be large in compari- 
son to the resistance of the meter. Adjust the variable resistor 
until the meter reads full scale. Then connect a small variable 
resistor in parallel with the meter and adjust until the meter 
reads one-half scale. Measure the resistance of the shunt added 
with an ohmmeter. This will be equal to the resistance of the 
meter within practical limits. 

1 



wwv 




FIGURE 1. Determination of ammeter resistance. 



• LntV 0I J' T ° CaHbrate an ammeter or milliammeter, 

£ Z2 \ aU a ? m ? Qr WhiCh is kn ° wn t0 be accurate in a circuit 
curSt i y J, aU ! °* CUrrent is fl0win * and measur e the 
cahhrl/ ,' SU ? St - Ute m the Same Circuit the amm «ter to be 
me er nf \no ^ ltS ^^ If n0t identical to tha * <* the 
vlded hv ST™ T? Cy ' make WhateWr ad J' us ^ents are pro- 
JotL J ^^ftarer, or note the correction that must be 

5K £ 7 g l taken Wlth this meter for futur e reference. 
Thxs method is sufficiently accurate for use by repairmen. 

10. EXTENDING AMMETER RANGE. To extend the range of 
an amme ter which has already been calibrated, a resistance of 
suitable value (shunt) is placed in parallel with the meter. If 
the meter resistance is known, the value of the shunt to be used 

R? V a /^T C, il ar range may be cal ™lated by the formula 
k - K-/(N-1), where R* is the resistance of the shunt, R™ is 
the resistance of the meter, and N is the number of times the 
range is to be extended. 

11. CLASSIFICATION OF AMMETERS. Current measuring 
Z^Z are + cIassified as DC or AC, and may be further classified 
meters 1 ".! f° ""^ "? milliamme ters. Ammeters and milliam- 
meters are frequently incorporated with voltmeters and ohmeters 
ZiZ Ultl "P Urp ? Se instrum ents. Care must be exercised when 
using such meters to assure use of the proper scale and the cor- 
rect setting of switches. 

12. AMMETER PRECAUTIONS. 

of a thrmeter neaSUring Unkn ° Wn currei *s, use the. highest range 

b. Observe proper polarity for the meter, 
^c. Do not connect a DC meter into a circuit where AC is flow- 

J^szzi e r meters before making changes in the -- 

SL? T» f RE t ?Y E f CY AMMET ERS. The basic meter move- 
Howevpr ™ satlsfactor y *» measuring RF current directly, 
flowfl ; I "X 7 be USed t0 heat a thermocouple, and current 
couBe g m n U \° ^ P ° tential difference devel °P ed in ^e thermo- 
sSce t r p y ^ T aS ^,f With a conventional meter movement, 
fcmce the potential difference developed in the thermocouple is 



directly proportional to the effective value of RF current the 
meter may be calibrated to measure accurately the flow of RF 
current. 

14 NEON LAMP. The presence of RF in a circuit may be dis- 
covered by touching a neon lamp to the circuit concerned. If 
RfTs present, the lamp should glow. If the circuit contains an 
demenrwhere lines of force are concentrated such as a c«l, 
theW need only be placed in the vicinity of this element. This 

est 3 Se satisfactory only where relatively large amounts of 
RF current are present, such as in most transmitter circuits. 

15 INCANDESCENT LAMP. The presence of DC or AC in a 
cfrcuVt may be discovered by placing an incandescent lamp n 
serie with the circuit. If current is flowing the lamp should 
Sow The size of lamp used must be governed by the amount 
o Turrent believed to be flowing. For very small currents a 
?lashhght bub should be used, and even this may give a false 

ndtatfon of no current. RF current in a coil may be ind j«b ed by 
connecting a- flashlight bulb in series with a loop of wire and 
inductively coupling the loop to the coil. 

SECTION III 

VOLTAGE MEASURING INSTRUMENTS 

16 CONSTRUCTION AND CONNECTION. Meters for meas- 
uring the voltage difference between two points may operate 
on the principle of the basic meter movement (par. 6) or may 
involve the use of a calibrated electron-ray tube. Vo tage meas 
urlng instruments must always be connected in parallel with the 
drcuit dement to be measured. The instrument resistance must 
always be high in comparison to that of the circuit element being 
tested. 

17 DETERMINATION OF VOLTMETER RESISTANCE. Con- 
nect a battery and meter in series so that the battery causes full- 
scale deflection on the meter. Add sufficient resistance in series 
to produce half-scale deflection, and measure the resistance 
added by use of an ohmmeter. This will be equal to the resist- 
ance of the meter within practical limits. 

18 a ^TbtJteT^ltmeter, connect a voltmeter which is known 
to be accurate across a circuit element through which a steady 



value of current is flowing and measure the voltage drop. Then 
substitute the voltmeter to be calibrated across the same circuit 
element and note its reading. If not identical to that of the ac- 
curate meter, make whatever adjustments are provided by the 
manufacturer, or note the correction that must be applied to 
readings taken with this meter for future reference. This 
method is sufficiently accurate for use by repairmen 

b. Measure the open circuit voltage of a fresh dry cell battery. 
This should be 1.53 volts. Calibrate the meter if possible, or note 
the correction that must be applied to the meter. For voltmeters 
with much greater ranges, use more than one dry cell battery, 
lnis method is sufficiently accurate for use by repairmen. 

19. EXTENDING RANGE OF VOLTMETER. To extend the 
range of a voltmeter which has already been calibrated, a re- 
sistance of suitable value (multiplier) is placed in series with 
the meter. If the meter resistance is known, the value of the 
multiplier to use for a particular range may be calculated by the 
tormula R = R™ (N-l), where R is the multiplier resistance, R,» 
is the meter resistance, and N is the number of times the range 
is to be extended. 

20 CLASSIFICATION OF VOLTMETERS. Voltmeters are class- 
ified as DC or AC, and may be further classified according to 
the internal resistance of the meter per volt. 

21. VOLTMETER PRECAUTIONS. 

a When measuring unknown voltages, use the highest range 
of the meter. 

b. Observe proper polarity for the meter. 

c Do not connect a DC meter into a circuit where AC is flow- 

; d. Disconnect the meter before making changes in circuit be- 
ing measured. 

e. Make sure that the meter resistance which is affecting the 
now of current in the circuit being measured is considered when 
interpreting results. 

22. VACUUM TUBE VOLTMETER. 

a General. DC vacuum tube voltmeters (VTVM) are partic- 
ularly useful because their input resistance (10 to 14 megohms) 



allows the meter to be connected to an operating stage of a 
radio set with negligible loading effect. 

b Circuit arrangement. A common type of vacuum tube volt- 
meter consists of a balanced bridge in which the plate resistance 
of a triode vacuum tube is one arm of the bridge, and a basic 
meter movement is connected across the bridge. The meter is 
made to read zero by adjusting the bias on the vacuum tube grid 
while the test prods are shorted together. The voltage to be 
measured is applied to the grid of the vacuum tube and the re- 
sulting apparent change in plate resistance of the vacuum tube 
unbalances the bridge and causes current to flow through the 
meter in proportion to the voltage being measured. 

c False zero. When small positive and negative voltages are 
to be measured, a VTVM may be set at false zero to facilitate 
work. With the meter actually measuring zero voltage (test 
prods shorted together), the grid bias is adjusted so that the 
meter pointer indicates some value greater than zero (e.g. mid- 
scale). If positive voltages are then measured, the meter will 
show an increase in deflection; and if negative voltages are 
measured, the meter will show a decrease in deflection 

d Improvised vacuum tube voltmeter. An improvised VTVM 
may be constructed as indicated in figure 2. The meter range 
is changed by changing the vacuum tube grid bias. Calibration 
does not have to be exact if used only for indicating voltage 
peaks However, if exact measurements are required, calibrate 
by measuring standard voltage sources. For accurate results, use 
a grid bias greater than the voltage to be measured. Plate cur- 
rent cut-off occurs at about minus 10^ volts bias. The base of 
a four prong tube may be used to connect to the battery BA-40. 
This meter may be used as a high resistance, low range milliam- 
meter if calibrated against a known value of current. 

VT-24 



INPUT TEST 




<7) 1-50 METER CONNECTED 
FOR 3 VOLT RANGE 



BOTTOM VIEW OE PLUG 

_90^ / + 90 TO PLUG INTO BA-40 

FIGURE 2. Improvised vacuum tube voltmeter. 



23. ELECTRON-RAY TUBE. 
• a. General. An electron-ray tube may be used to indicate volt- 
age peaks or dips, and, if calibrated, may be used to measure 
voltages within fairly broad limits. This instrument has a 
high input resistance and may be connected to an operating stage 
with negligible loading effect. 

b. Circuit arrangement. A common type of electron-ray tube 
circuit consists of a triode and electron-ray indicator in one glass 

coSVl 1 ? CatG i in fi * Ure 3 - The voIta * e t0 be meas »red is 
connected between the triode grid and cathode so that larger 
voltages will make the grid more negative. As the grid becomes 
more negative, the triode plate current becomes smaller, the 
collector plate becomes more nearly same potential as the electron- 
ray target, and the dark section or shadow of the target becomes 
smaller. This instrument may be calibrated by applying a 
standard voltage to the triode grid and adjusting the plate 
voltage or grid bias so that the shadow just closes when that 
voltage is measured. 



TO VOLTAGE 

TO BE •« — vwwvv 

MEASURED R, 




FIGURE 3. Electron-ray tube application. 

24. INCANDESCENT LAMP. The presence or absence of a volt- 
age between two points can be indicated by connecting an mean- 



descent lamp between those points. The lamp will glow if a volt- 
age is present. The size of lamp must be carefully chosen accord- 
ing to the voltage expected or this test will P r ° duc \™ sle t dl1 ^ 
results; for example, for small voltages a flashlight bulb should 

be used. 

25 HEADPHONES AND CONDENSER. The presence or absence 
of' a voltage between two points may be indicated by connecting 
headphones in series with a .01 microfarad condenser across the 
points where a voltage is to be measured. A click indicates the 
presence of a voltage and the loudness of the click indicates the 
relative magnitude of the voltage. This test should not be used 
for potentials over 100 volts. 

SECTION IV 
POWER MEASURING INSTRUMENTS 

26 GENERAL. Electrical power consumed by a circuit requires 
that both current and voltage be considered. This can be done in 
DC circuits by measuring current using an ammeter and voltage 
using a voltmeter, and multiplying the two results to obtain 
power in watts. Power measurements are relatively unimpor- 
tant to the radio repairman except in aligning or testing receiv- 
An output meter may be used for this purpose. 



ers. 



27. OUTPUT METER. 

a. General. An output meter is designed to take output read- 
ings directly from the output circuit of any vacuum tube A 
series condenser is provided to isolate the instrument so that 
DC potentials will not cause damage to the instrument. 

b Circuit arrangement. A common type of output meter em- 
ploys a full wave bridge rectifier system and a basic meter move- 
ment which indicates power by measuring the current flowing 
through a standard resistor contained within the instrument. 
The total impedance across the input terminals is made constant 
on all ranges of the instrument so that the loading of the output 
tube will not be changed when the meter range is changed. 

SECTION V 

CONTINUITY INDICATORS 

28 GENERAL. The continuity of a circuit or element of a cir- 
cuit may be determined by means of a continuity tester. Its 

10 



essential parts are a source of power to provide current when 
the circuit is complete, an indicator to indicate the flow of cur- 
rent *and associated wiring and test prods. The actual resistance 
o± a circuit may or may not be measured. 

29. OHMMETER. 

a. General. A series type ohmmeter is indicated in figure 4 
The circuit is designed so that the battery can be adjusted to 
cause full-scale deflection of the meter when the test prods are 
shorted together. When the test prods are connected to an un- 
known circuit, the meter pointer will move down scale an amount 
proportional to the resistance of the circuit being measured. If 
the circuit is open or has extremely high resistance, the pointer 
will drop to indicate infinite resistance. The meter is normally 
calibrated in ohms. 

b. Extending range. The range of an ohmmeter may be doubled 
by connecting sufficient external resistance into the circuit to 
cause the meter to read one-half scale and then adding sufficient 
batteries to cause the meter again to read full scale. 





x/VWVW-r 



FIGURE 4. Series type ohmmeter. 

30 HEADPHONES AND BATTERY. An elementary continuity 
tester is indicated in figure 5. When test prods are placed across 
a low resistance, a loud click will be heard in the headphones. 
A high resistance will produce faint clicks. More than 25 milli- 
amperes flowing for a long period will damage the headphones. 

31. LAMP AND BATTERY. An incandescent lamp may be sub- 
stituted for the headphones in paragraph 30. This arrangement, 
however, is unsatisfactory for high resistances. 

11 



INPUT 





FIGURE 5. Elementary continuity tester. 



7 

B + 



32. VOLTMETER AND BATTERY. A voltmeter may be sub- 
stituted for the headphones in paragraph 30. If the battery 
causes full-scale deflection when the test prods are shorted to- 
gether, low values of resistance will be indicated near full-scale 
deflection, and high values near zero deflection. By comparing 
readings obtained with unknown resistors to those obtained 
with resistors of known value, actual resistances of unknown 
elements may be closely approximated. 

SECTION VI 
SIGNAL SOURCES 

33. GENERAL. For testing and aligning radio receivers, signals 
of known frequency are required. These may be either modulated 
or unmodulated — usually the latter. 

34. OSCILLATOR. A stable oscillator circuit, preferably crystal 
controlled, will provide a CW signal of constant frequency. This 
signal may be coupled directly to that portion of the radio set 
to be tested. A control is normally available so that the volume 
of the signal may be controlled. The manufacturer's instruction 
book should be consulted for details concerning construction, use, 
and maintenance of commercial oscillators. 

35. HETERODYNE FREQUENCY METER. A heterodyne fre- 
quency meter is designed to measure or to radiate radio frequency 
CW signals on a wide range of frequencies. When used as a 
signal generator it should be coupled to the radio receiver through 
a .001 microfarad condenser to prevent undesired loading effects. 
The frequency meter should be used only when accurately cali- 

12 



brated. The manufacturer's instruction book should be con- 
sulted for details of construction, use and maintenance. 

36. TRANSMITTER. A good transmitter capable of producing 
a signal of constant frequency may be used as a signal source. 
Power amplifiers should be disabled where possible to reduce 
output. If a modulated signal is required, this may be obtained 
by talking or whistling into the microphone. If the transmitter 
is crystal controlled and uses frequency multiplication, a wide 
variety of radio frequency signals may be obtained by selecting 
the proper . crystal and harmonic to give the output frequency 
desired. Care is necessary to make certain that the correct 
harmonic is obtained. This may require the use of a frequency 
or wave meter. 

37. RECEIVER AND TRANSMITTER. If a signal is required 
for aligning receiver stages, a good transmitter may be operated 
so that its signal is received on a good receiver of same type as 
the receiver under test. Connect the chassis of both receivers 
together and feed an RF, IF or AF signal as required from the 
good receiver to receiver under test by connecting corresponding 
grids of the desired stage together using a lead wire with a series 
condenser. 



SECTION VII 
SIGNAL TRACER 

38. GENERAL. Signal tracers are frequently used to trace the 
progress of a radio signal through a receiver, and locate troubles. 
Signal voltages developed at any point in the receiver are trans- 
ferred to the signal tracer as indicated below and the quality of 
the signal at that point is noted by listening at the reproducer 
of the signal tracer. Usually, the signal is traced first from the 
antenna of the receiver being tested and then progressively 
at various points to the reproducer of the receiver being tested. 

39. AF SIGNAL TRACER. Audio frequency signals may be 
traced using conventional headphones in series with a condenser 
to prevent passage of DC through the headphones. Connect 
the headphones across any impedance where an audio voltage 
is developed. 

13 



40. UNTUNED SIGNAL TRACER. A schematic diagram for an 
improvised, untuned signal tracer capable of tracing signals 
through RF and IF sections, as well as through AF sections, is 
shown in figure 6. This circuit will respond to a wide range of 
frequencies, but will not indicate the frequency of the signal and 
is not satisfactory with frequency modulated signals. Connect 
the proper input terminals indicated in figure 6 across any im- 
pedance where an RF or AF voltage is developed and listen at 
the output of the signal tracer. 

SHIELD (GROUNDED) —> 



AF 



COMMONl 




^ ,|p — 4*!l'.i 



FIGURE 6. Untuned signal tracer. 

41. GOOD RECEIVER. A receiver known to be good may be used 
as a signal tracer. Connect the chassis of a good receiver and 
the receiver being tested together and then transfer signals 
from the receiver being tested to the good receiver by con- 
necting corresponding points in the desired stage of each, using 
a lead wire with a .001 microfarad series condenser. 



14 



CHAPTER 3 
TESTING RADIO PARTS AND ASSEMBLIES 

Paragraphs 

Section I. Fundamental parts 42-48 

II. Special units 49-68 

SECTION I 
FUNDAMENTAL PARTS 

42. GENERAL. Radio sets contain some or all of the following 
fundamental parts: resistors, condensers, coils, transformers, 
tubes, and electrical connections. Appropriate tests are given 
below for these parts. Unless otherwise indicated, all tests are 
for parts not connected in a circuit. 

43. RESISTORS. 

a. Classification. Resistors are either fixed or variable and are 
characterized by their ohmic resistance and wattage rating. 
They may be classified according to usage as: voltage dropping, 
current limiting, coupling, decoupling, filter, or load resistors. 
Resistors may develop opens or shorts and may change resistance. 

b. Open. Open resistors may be discovered by testing with any 
appropriate continuity tester. If the resistor is open the con- 
tinuity tester will indicate no flow of current. 

c. Shorts. Shorted resistors may be detected by use of an ohm- 
meter. The ohmmeter will indicate zero resistance, or if only 
partially shorted, the ohmmeter. will indicate a resistance be- 
tween zero and the rated value. 

d. Resistance changed. Resistors which have permanently 
changed in value may be detected by use of an ohmmeter. How- 
ever, during operation of radios, resistors sometimes make er- 
ratic changes in value. This is evidenced by noisy operation in 
the reproducer. Figure 7 shows a circuit which may be used to 
check resistors for noisy operation. 

44. CONDENSERS. 

a. Classification. Condensers are either fixed or variable and 
are characterized by their capacity, working voltage, surge volt- 

15 



Resistor under test 

(1000 OHMS OR MORE) 



II H 



TO AF 
AMPLIFIER 



200 V 

FIGURE 7. Circuit for testing noisy resistors. 

age, and dielectric. They may be classified according to usage 
as: coupling, decoupling, bypass, DC blocking, filter, tuning, 
trimmer, and padder condensers. Condensers may develop opens, 
shorts, or leakage (high resistance short), and may change ca- 
pacity. Electrolytic condensers should be tested by substitution 
since no other method is quite satisfactory. Condensers with air, 
paper, or mica dielectric may be tested as indicated below. 

b. Working voltage test. Connect a DC power supply in series 
with a meter capable of measuring its highest voltage and test 
the condenser under its rated working voltage as indicated in 
figure 8. If the capacity of the condenser is more than .01 micro- 
farad, failure to obtain an initial surge on the meter will indicate 
an open condenser. If the meter does not return to zero, a short 
is indicated. 




I 



FIGURE 8. Condenser working voltage test. 

c. AC voltage test. Connect an AC power supply and an AC me- 
ter capable of measuring its highest voltage in series and test 
condenser as in figure 8. Zero meter reading indicates an open 
condenser. Full voltage of the power supply indicates a shorted 
condenser. A good condenser will give a reading somewhat be- 



16 



low full voltage. An AC voltmeter and 110 volt AC power sup- 
ply may be used with condensers of known capacity to establish 
a comparison chart for determining unknown capacities. For 
condensers with capacities above .5 microfarad, a 15 watt lamp 
may be substituted for the meter and its brilliance used to indi- 
cate the condition of the condenser. Condensers to be tested 
with AC must be removed from the radio set. 

d. Charge and discharge test. Condensers with capacities of 
less than .1 microfarad may be charged using a 3 volt battery 
and then discharged through headphones. A loud click indicates 
a good condenser, a weak click indicates leakage, and no click 
indicates a short or open. For larger condensers a 90 volt battery 
should be used. After one minute the condenser should be dis- 
charged by shorting with a screwdriver and the spark strength 
used to judge the condition of the condenser. 

e. Ohmmeter test. Low capacity condensers may be tested 
using an ohmmeter. When connected to an ohmmeter, a good con- 
denser will give an initial meter surge and then indicate infinite 
resistance. A shorted condenser will indicate a low resistance. 
An open condenser will give no initial surge on the meter. This 
method is not conclusive because it will not indicate a high re- 
sistance short. 

45. COILS. 

a. Classification. Coils may be fixed or variable and are char- 
acterized by their inductance and type of core. They may be 
classified according to usage as : RF choke, AF choke, filter, 
tuning, and load coils. Coils may develop opens, shorts, or 
grounds. 

b. Opens. Any form of continuity tester may be used to check 
for open coils. If the coil is open, the continuity tester will indi- 
cate no flow of current. 

c. Shorts. Shorts are difficult to locate by electrical test. RF 
coils should be inspected visually for shorts and AF coils may be 
checked by an ohmmeter if the resistance of the good coil is 
known. 

d. Grounds. RF coils are usually wound on insulated forms and 
are not apt to have grounds. Coils using metallic cores may be 
checked for grounds using a continuity tester as in figure 9. If 
the coil is grounded, the continuity tester will indicate a flow 
of current. 

17 



46. TRANSFORMERS. 

a. Classification. Transformers are characterized by their turns 
ratio, number of secondaries, type of core, and impedance. They 
may be classified according to usage as: RF, IF, AF, or power 
transformers. Transformers may develop opens, shorts, or 
grounds. Power transformers may have secondaries improperly 
connected. 

b. Opens. Any form of continuity tester may be used to check 
for open transformers (figure 9). If the transformer winding 
is open, the continuity tester will indicate no flow of current. If 
the open is caused by "burning out", causes of high current must 
be investigated. RF transformers usually develop opens only. 

c. Shorts. Shorts may be tested as indicated in figure 9. _.If 
the transformer windings are shorted, the continuity tester will 
indicate a flow of current. Shorts at radio frequencies only may 
be difficult to discover except by visual inspection or by substi- 
tution. 



CHECK 
TO. FOR 
J=2 OPEN 



I 



I 



Hi|h-^) 



CHECK FOR 
GROUND 



CHECK FOR SHORT 

FIGURE 9. Testing transformer with continuity indicator. 

d. Grounds. Grounds may be tested as indicated in figure 9. If 
the transformer winding is grounded, the continuity tester will 
indicate a flow of current. Grounded secondaries are frequently 
the cause of burned-out primaries, and are often indicated by 
heat, smoke, or unusual odors. 

e. Identification of leads. The leads of power transformers 
which are not marked may be identified by placing each winding 
successively in series with a 60 watt lamp and 110 volt AC power 
supply. The primary winding will allow the lamp to glow faintly, 
the high voltage secondary winding will produce only a faint 
spark, and the low voltage secondary will allow the lamp to glow 
brilliantly. 



18 



47. TUBES. 

a. Classification. Tubes may be high vacuum or gas filled, and 
are characterized by the number and arrangement of their ele- 
ments, type of envelope, type of base, socket connections, and 
electrical characteristics. Tubes may be classified according to 
use as: oscillator, amplifier, detector, modulator, rectifier, volt- 
age regulator, frequency converter, or electron-ray tubes. One 
tube may perform more than one function if suitable circuit 
changes are made. Open filaments, shorted elements, low emis- 
sion (weak), open connections, cathode to heater leakage, and 
gassy tubes may cause partial or complete failure of tubes. De- 
fective tubes are best discovered by substitution although num- 
erous other tests are possible. When testing by substitution, al- 
ways place the suspected tube in a good receiver or transmitter 
and check performance. This will eliminate the chance of ruin- 
ing numerous good tubes by placing them in a defective radio set 
which may burn out their filaments, and will make trouble loca- 
tion easier when more than one trouble exists. 

b. Base connections. Tube elements are brought from inside the 
tube to external connections at base prongs or a cap on top of 
the envelope. Schematic diagrams of radio sets indicate the 
prong number at which each tube element is terminated. A stand- 
ard system used for numbering prongs viewed from the bottom 
of the tube base is indicated in figure 10. Seven-prong tubes 
which fit eight-prong sockets are numbered the same as the 
eight-prong except that the number of the missing prong is 
omitted. 



FOUR PRONG 
TYPE 



FIVE PRONG 
TYPE 



EIGHT PRONG 
TYPE 






KEY 



FIGURE 10. Tube base numbering system (bottom view). 

c. Open filament. A continuity indicator will test for open 
filaments. 

19 



d. Shorted elements. If two elements of a tube are shorted to- 
gether, a continuity indicator connected between external con- 
nections of these two elements will indicate a continuous path 
for current. Commercial tube testers usually incmde a short- 
circuit test of this nature. 

e. Low emission. An operational test in a good radio set is the 
best test for low emission. Commercial tube testers provide an 
emission test in which all electrodes except the cathode are con- 
nected to the plate. The filament or heater is operated at rated 
voltage, and a low positive voltage is applied between the plate 
and cathode of the tubes. Relative electron emission is indicated 
on a series milliammeter. Low emission indicates that the tube 
is approaching the end of its useful life. 

f. Open connections. Open connections are easiest to discover by 
substitution of the tube in a good radio set. 

g. Cathode to heater leakage. Cathode to heater leakage is a 
form of short circuit and may be discovered in a similar manner 
using a commercial tube tester. 

h. Gassy tubes. High vacuum tubes sometimes fail due to ac- 
cumulation of gas in the envelope. This may be indicated by a 
tube tester, or a special test may be made for resistance coupled 
receiver amplifier tubes. With the suspected tube in a good re- 
ceiver and receiving no signal, connect a vacuum tube voltmeter 
to measure grid voltage. A positive voltage on the grid will indi- 
cate a gassy tube if the coupling condenser from the preceding 
stage is not leaky. A gassy tube may be distinguished from a 
leaky coupling condenser because the latter gives a positive volt- 
age immediately after the set is turned on, while the former must 
first warm up, and this may require several minutes. If in doubt, 
disconnect one side of the coupling condenser and repeat the test. 

i. Transconductance test. Most commercial tube testers incor- 
porate a test to determine the transconductance of the tube. Rated 
DC voltages are applied to each element of the tube. For a static 
test, the grid bias is changed and the corresponding change in 
DC plate current indicates the transconductance of the tube. For 
a dynamic test, an AC voltage is applied to the grid and the 
changing plate current indicates the transconductance of the 
tube. 

j. Power output test. Commercial tube testers may include a 
power output test. Power developed in a plate load when an AC 
signal is impressed on the grid is determined. This provides an 
excellent test to determine the operating condition of the tube. 

20 



48. ELECTRICAL CONNECTIONS. Electrical connections in ra- 
dio sets are of great importance and may cause a high percentage 
of all troubles. Defective connections may be discovered by visual 
inspection and by use of an ohmmeter. Connections should be 
mechanically fastened to sockets and terminals, and then sol- 
dered. Soldered connections made with a cold iron (resin joints) 
pull loose easily and may have high resistance as measured with 
an ohmmeter. 

SECTION II 
SPECIAL UNITS 

49. GENERAL. Many special units or assemblies are used in radio 
sets. Tests indicated in this section are for units when not con- 
nected to radio sets unless specifically indicated otherwise. 

50. CORDS, PLUGS, AND JACKS. A large proportion of all 
troubles in radio sets is due to defective cords, plugs, and jacks. 
Defects may be opens, shorts, grounds, defective connections, and 
corroded contacts. All defects may be discovered by means of 
visual inspection and testing with a continuity tester. The re- 
sistance of cords, plugs, and jacks should approach zero for each 
particular circuit. Electrical connections should be satisfactory, 
insulation undamaged and complete, conductors not grounded to 
shielding except when designed to be grounded, plugs and jacks 
fitted tightly, socket pins straight and well burnished, keys and 
keyways smooth and undamaged, and all sliding contacts well 
burnished. 

51. SWITCHES. Most switches used in field artillery radio sets 
are mechanically operated and use.. sliding or knife contacts. 
Switches may develop opens, shorts, grounds, and high resistance 
contacts, and may be physically damaged so that proper contacts 

> are not made. Visual inspection and testing with a continuity 
indicator should disclose all defects. Switches must be kept free 
from 'water and lubricants. 

52. SHIELDING. In high frequency radio stages having high 
pin, shielding is employed to prevent undesired feedback. The 
input circuit, output circuit, and the tube should be shielded from 
each other and from other high frequency stages. Coils are usu- 
ally mounted in shield cans, ganged tuning condensers usually 
nave baffle plates mounted between sections, and tubes have 



&37??7- 



21 



metal shields which slip over the tube envelope. All shields 
should be grounded to the chassis by short and heavy connec- 
tions. Metal tubes require no shielding. Lead wires may some- 
times be shielded by enclosure in metallic sheathing which is 
grounded to the chassis. Shields should never be removed unless 
immediately replaced. Visual inspection and electrical checks of 
ground connections should adequately test shielding. 

53. FUZES. Fuzes are used in radio sets to protect meters, tubes, 
and other equipment from excessive currents. Fuzes must never 
be short circuited. Whenever fuzes burn out, the cause of ex- 
cessive current must be discovered and eliminated. Open fuzes 
may be discovered by visual inspection or by using a continuity 
tester. 

54. CIRCUIT BREAKERS. Circuit breakers serve a function sim- 
ilar to that of fuzes and must never be tied down or short cir- 
cuited. 

55. RELAYS. Relays are used to complete electrical circuits and 
to regulate current and voltage of power supplies. When used 
to complete electrical circuits, the relay normally consists of an 
electromagnet and armature. A low voltage circuit energizes 
the electromagnet when a key or switch is operated, and the 
electromagnet attracts the armature thereby making or break- 
ing one or more electrical contacts. The electromagnet coil may 
develop opens, shorts, or grounds and may be tested in the same 
manner as other coils. Tension of the armature restoring spring, 
the armature gap distance, and mechanical operation of the arm- 
ature may be checked by visual or mechanical inspection. Pitted 
or sticking points may be checked by visual inspection or by not- 
ing irregular operation of the relay and arcing at the points. 
Relay points should never be touched by the fingers, as perspira- 
tion or oil will cause defective operation. 

56. CRYSTALS. Quartz crystals used to control the frequency of 
oscillators are relatively free from trouble if carefully handled. 
Rough treatment may cause crystals to crack or connections to 
break. Crystals should be tested by installing in an oscillator 
circuit and measuring the rectified grid voltage developed while 
oscillating with a VTVM. Voltage measured should be compared 
to that of a crystal known to be good when used in the same cir- 
cuit. The rectified grid voltage indicates the relative activity of 
the crystal, and in most applications will be negative 41/2 volts 
or more. 

22 



57. COPPER OXIDE RECTIFIER. Copper oxide rectifiers may be 
tested by impressing, in turn, DC voltages of opposite polarity 
on the rectifier and measuring the current flowing with a milli- 
ammeter. Current flowing should be high in one case and very 
low in the other case. 

58. THERMOCOUPLES. Thermocouples used in connection with 
the measurement of RF current may burn out due to currents 
greater than rated value flowing through the thermocouple. The 
most satisfactory method for testing is to substitute a good ther- 
mocouple. To prevent burning out, the position of thermocouple 
should be checked and changed if necessary to reduce the amount 
of current flowing through it. 

59. METERS. Meters when used as part of a radio set may de- 
velop open or shorted coils in the movement, frozen bearings, 
broken glass, broken pointer, or may change calibration. De- 
fects should be detected by visual inspection, by measuring volt- 
age or current from a standard source, or by comparison with 
meters known to be good. If calibration is defective, the meter 
may be recalibrated by comparison with good meter (see pars 
9 and 18). 

60. ANTENNA. Antennas may develop opens, shorts, or grounds ; 
connections may become corroded; capacity grounds may exist; 
and the electrical length may be incorrect. Opens, shorts, and 
grounds will cause abnormally high or low antenna current when 
the transmitter is operating. Defective connections may be de- 
tected by visual inspection. Capacity grounds occur occasionally 
on vehicular installations. They may be caused by antennas be- 
ing tied down with insulated wire, failure to remove coaxial con- 
necting wire from the mast base when an ordinary lead-in is 
used, installation of the antenna too close to metal bows in the 

v top of the vehicle, etc. Capacity grounds detune the antenna and 
may be detected by visual inspection and by trial and error 
changes. The physical length and construction of an antenna is 
specified by appropriate technical manuals for each installation 
and must be used exactly as specified. Any change in physical 
nature will change the electrical length and thereby detune the 
antenna. 

61. DRY CELL BATTERIES. Dry cell batteries deteriorate with, 
use and with age. Defective dry ceil batteries may be detected 

23 



by measuring the output voltage under normal load after the 
set has been turned on for at least five minutes. If the voltage 
is less than 80% of rated voltage, the battery should be replaced. 

62. STORAGE BATTERIES. Vehicular storage batteries must be 
kept well charged at all times. The state of charge may be check- 
ed by measuring the specific gravity of the battery electrolyte 
with a hydrometer. For batteries used in temperate or cold 
climates, a fully charged battery will test between 1.275 and 
1.300 at 60° F. For batteries used in torrid climates, a fully 
charged battery will test between 1.205 and 1.230 at 60° F. To 
correct actual specific gravity to 60° F, add .004 to the gravity 
reading for each ten degrees of electrolyte temperature above 
60° F, and deduct .004 for each ten degrees below 60° F. 

63. GENERATORS. High voltages may be obtained by means of 
hand or motor driven generators. These generators usually con- 
sist of an armature, a set of field coils with permanent magnet 
poles, and one or more commutators. The output windings are 
wound on the armature and terminate at segments of the com- 
mutator. The output voltage is removed from the commutator 
through a pair of carbon brushes and delivered to the radio set 
through a filter system. The output voltage is produced by rota- 
tion of the armature in a magnetic field. Common causes of 
trouble are: dirty generator; improper lubrication; worn or 
damaged brushes or brush holders ; open, short or grounded field 
coils; open, short, or grounded armature; shorted or damaged 
commutator; open, short, or grounded filter system; or defective 
electrical connections. Visual inspection and tests of individual 
parts for shorts, opens, and grounds will indicate defects. 

64. DYNAMOTORS. Dynamotors are similar to generators except 
that a low voltage commutator and winding are added to the 
armature so that the armature may be driven by means of a 
storage battery. Troubles and tests are identical. 

65. VIBRATORS. Vibrators are used to feed pulses of current 
from a storage battery through the primary of a power trans- 
former. Construction and operation is similar to that of relays. 
Low voltage from the battery is particularly hard on vibrators 
and will cause the points to stick. Vibrators are easiest to test 
by substitution in a power supply known to be good. When the] 
input circuit is closed, operation of the vibrator may be checked 

24 



by placing the fingers on the vibrator container. Vibration of 
a good unit can be felt. 

66. HEADPHONES. A headphone consists of a diaphragm sup- 
ported in an insulated shell, a permanent magnet, and two coils 
connected in series as indicated in figure 11. Audio frequency 
current in the coils causes their magnetic fields alternately to 
aid and oppose the field of permanent magnet, thereby causing 
the diaphragm to vibrate at an audio rate. A headphone unit 
may develop opens or shorts in the coil which may be detected 

DIAPHRAGM 



HARD RUBBER 
HOUSING 




PERMANENT 
MAGNET 

FIGURE 11. Headphone unit. 

with a continuity tester. The diaphragm may become bent or 
broken or may be assembled incorrectly. Cords and plugs and 
electrical connections should be checked. 

67. LOUDSPEAKER. A permanent magnet, dynamic loudspeaker 
is shown in figure 12. Audio frequency current in the voice coil 
causes its magnetic field alternately to aid or oppose the field of 
a permanent magnet, thereby causing the voice coil and cone to 
move back and forth at an audio rate. Opens, shorts, or grounds 
in the voice coil may be located using a continuity tester. If the 
voice coil is improperly centered, faulty reproduction or 
"scratchy" noises will be heard, and the voice coil must be care- 
fully recentered by adjusting the location of the spider. Defects 
m the paper cone may be noted by visual inspection. 

68. MICROPHONE. The carbon button microphone T-17 is used 
almost universally in field artillery units to convert sound into 
electrical energy. Its circuit is shown in figure 13. The carbon 
button is connected in series with the ring and sleeve of the plug 
when the microphone switch is closed. The plug tip and sleeve 
circuit, completed through the microphone switch, may be used 



25 



CONE 



••CDinCD" 




PERMANENT MAGNET 



FIGURE 12. Permanent magnet, dynamic speaker. 

to control operation of relays and power supplies in radio sets. 
Opens or shorts may be detected with a continuity tester. The 
ring-sleeve circuit should have 100-150 ohms resistance with the 
microphone switch depressed. Blowing into the microphone 
should vary this value. The tip-sleeve circuit should have 0-.5 
ohms resistance with the microphone switch depressed. 



MICROPHONE 



\ 



o 



INSULATING SHELL 



TIP 



= TTr 

rty-'tA 



vmiiWttMMItMifo 



\ 

SLEEVE 



MHO 

t 
RING 



SWITCH 

FIGURE 13. T-17 Microphone circuit. 



26 



CHAPTER 4 
TESTING CIRCUITS 

Section I. Fundamental vacuum tube circuits Paragraphs 

tJt Special vacuum tube circuits 75-82 

HI. Power supply circuits , 83-90 

SECTION I 
FUNDAMENTAL VACUUM TUBE CIRCUITS 

69. GENERAL. When trouble has been isolated to a particular 
stage, individual circuits which make up that stage should be 
tested. Unless experience indicates the cause of trouble imme- 
diately, circuits should be checked in the following order: fila- 
ment, plate, control grid, screen grid, and suppressor grid. These 
circuits sometimes include, or function with, special circuits in- 
dicated in section II. A circuit diagram of the radio equipment 
being tested should be consulted as tests are made in order to 
determine peculiarities of each particular circuit. 

70. FILAMENT CIRCUIT. The filament circuit heats the cathode 
of the tube. This circuit will contain some or all of the following 
components : tube cathode or heater, power supply, switch, rheo- 
stat, cathode resistor, shunt resistors, fuze, voltage dropping 
resistors, and other tube cathodes or heaters in series or in 
parallel. Trouble may be caused by defects in any component or 
in wiring and connections. Test the power supply to determine 
the presence of rated voltage; test the tube for open filament 
shorted elements, improper type, or improper insertion in socket; 
test the entire circuit for continuity; and, test individual parts 
for opens, shorts, grounds, or changed values. 

71. PLATE CIRCUIT. The plate circuit provides plate current 
for the stage and a load to develop the output voltage or power. 
This circuit will contain some or all of the following components: 
tube, power supply, plate load, voltage dropping resistors, cathode 
resistor, tuned circuit, coupling device, or decoupling device. 
Trouble may be caused by defects in any component or in wir- 

27 



ing and connections. Test the power supply for rated voltage; 
test tube for open or shorted elements, improper type, improper 
insertion in the socket, gas in the envelope, or low emission; test 
the entire circuit from plate to cathode for continuity and total 
resistance ; and test individual parts for opens, shorts, grounds, 
or changed values. 

72. CONTROL GRID CIRCUIT. The control grid or input circuit 
controls the flow of plate current through the tube. This circuit 
will contain some or all of the following components : tube, bias 
supply, grid leak resistor and condenser, cathode resistor, grid 
load, tuned circuit, coupling device, or volume control. Trouble 
may be caused by defects in any component or in wiring and con- 
nections. Test the bias supply for rated voltage ; test the tube for 
open or shorted elements, improper type, improper insertion in 
the socket, or gas in the envelope; test the entire circuit from 
grid to cathode for continuity and total resistance; and, test in- 
dividual parts for opens, shorts, grounds, or changed values. 

73. SCREEN GRID CIRCUIT. The screen grid circuit reduces 
the inter-electrode capacity between the plate and the control 
grid, and makes plate current relatively independent of small 
changes in plate voltage. Voltage is obtained from the same 
source as plate voltage but is usually somewhat lower. This cir- 
cuit may contain some or all of the following parts : tube, power 
supply, voltage dropping resistors, cathode resistor, or decou- 
pling device. Trouble may be caused by defects in any component 
or in wiring and connections. Test in the same manner as for 
plate circuits. An open screen grid circuit may cause the plate 
current to drop almost to zero. 

74. SUPPRESSOR GRID CIRCUIT. The suppressor grid circuit 
is usually used to prevent secondary emission, but may be used 
for other purposes. When connected to the cathode inside the 
tube, no tests are possible. When used for modulation, the cir- 
cuit will be similar to the control grid circuit and may be tested 
in the same manner. 

SECTION II 
SPECIAL VACUUM TUBE CIRCUITS 

75. GENERAL. Special vacuum tube circuits are generally used 
in connection with or as a part of fundamental vacuum tube cir- 

28 



cuits. Circuit diagrams must be consulted while testing special 
circuits in a particular radio set. 

76, COUPLING CIRCUITS. 

a. General. Coupling circuits cause a transfer of voltage or 
energy from one stage to another due to a change of current in 
the first stage. Various types of coupling between vacuum tube 
stages are indicated in figure 14. 

RESISTANCE IMPEDANCE TRANSFORMER 




W (b) 

FIGURE 14. Coupling systems. 



(c) 



b. Resistance coupling. Troubles in resistance coupling circuits 
may be caused by defects in any component or in wiring and 
connections. If the resistors or the coupling condenser change in 
value, the frequency response may change. To test for a leaky 
coupling condenser while the receiver is operating, measure the 
grid voltage with no signal using a VTVM. Normal voltage is 
zero. If the coupling condenser is leaky, a portion of the DC 
Plate voltage from the preceding stage will be impressed on the 
grid and will cause a positive voltage to be indicated. 

c. Impedance coupling. Troubles in impedance coupling cir- 
cuits may be caused by defects in any component or in wiring 
and connections. In some cases, leaky coupling condensers may 
be detected as in b. above. 

d. Transformer coupling. Troubles in transformer coupling are 
those common to any transformer (par. 46), except that when 
the transformer windings are a part of resonant circuits addi- 
tional troubles may arise. If the turns ratio of audio frequency 
transformers is changed, the response at high or low frequencies 
will be materially affected. Resonant coupling circuits may be 
tuned independently if the coupling is loose, but if the coupling 
is at the critical or greater degree, the tuning of one circuit will 
affect the tuning of the other. Under-coupling results in low 
transfer of power from one circuit to the other, and over-coupling 
results in broad frequency response and reduced sensitivity. To 

29 



tune and couple two resonant circuits properly, proceed accord- 
ing to the set manufacturer's instructions. 

77. FILTER CIRCUITS. Filter circuits are used to prevent or 
reduce fluctuations in DC potential from various causes. These 
circuits consist of one or more choke coils or resistors in series 
with the potential to be filtered and one or more condensers in 
parallel. Troubles may be caused by defects in any component 
or in wiring and connections. Test continuity of the circuit and 
test individual parts for opens, shorts, grounds, or changed value. 
If the filter circuit to be tested is a part of a power supply, or 
other unit, the only test which will positively indicate that it is 
good or bad is to substitute the power supply or other unit for 
one which is known to be good, and compare its operation with 
that of the good unit. 

78. DECOUPLING CIRCUITS. Decoupling circuits isolate indi- 
vidual stages from each other when a common power supply is 
used. These circuits are commonly used as a part of plate and 
screen grid circuits, and usually consist of a suitable resistor in 
series with the power supply and a condenser in parallel. Compo- 
nents may be tested for open, shorts, grounds, and changed 
values. 

79. BYPASS CIRCUITS. Bypass condensers are used to provide 
a low impedance path for AC around a source of DC potential. 
The condenser should be tested for opens and shorts. If the j 
screen grid bypass is shorted, plate voltage will decrease; and 
if it is open, amplifiers may oscillate. 

80. MANUAL VOLUME CONTROL. The signal output of radio 
receivers may be varied manually by means of a potentiometer j 
across the load of an AF stage, or across the cathode resistor of j 
an RF or IF stage when a variable-mu tube is used. In either [ 
case, the potentiometer becomes a part of the grid circuit and 
is tested as any variable resistor would be tested. 

81. AUTOMATIC VOLUME CONTROL (AVC). 

a. General. AVC provides relatively constant signal level from j 
radio receivers regardless of changes in signal input due to fad- J 
ing, etc. A typical system is shown in figure 15. If the' signal, 
strength increases, negative voltage at the diode load increases f 
and causes a more negative bias to be placed on the grids of RF 

30 



or IF amplifiers. A time-delay filter is required to remove audio 
variations from the bias voltage. 

b. Troubles and tests. Any component of an AVC circuit or 
its wiring and connections may be defective. If the AVC circuit 
is not operative, though the receiver is otherwise normal, a large 



OUTPUT 
OF LAST 
IF STAGE 



AVC BIAS'" 

VOLTAGE TO 
RF OR IF 
CONTROL GRIDS 




A. .R... ,B i 
\J1 TIME DELAY 
'-T- FILTER 



C s 



TO AF 
^STAGE 



FIGURE 15. Typical AVC system. 



increase in volume is noted when tuning from a weak to a strong 
signal. To isolate the trouble proceed as follows: . 

(1) Measure the voltage from control grid to ground on 
each stage affected by the AVC system, using a vacuum tube 
voltmeter while the receiver is tuned to a signal. A reading of 
minus one volt or more should be indicated, depending on the 
strength of the signal. 

(2) If the voltage is correct on some stages and not on 
others, check the circuit between the grid of the defective stage 
and A of figure 15. 

(3) If the voltage is incorrect on all stages, check the volt- 
age between A and ground. This is the last point common to all 
stages. 

(4) If the voltage is incorrect at A, check between B and 
ground. If the voltage is correct at B, the time-delay filter is de- 
fective. 

(5) If the voltage is incorrect at B, check the voltage at 
the diode load resistor where the AVC voltage first appears. 



31 



82. DELAYED AUTOMATIC VOLUME CONTROL (DAVC). 

Negative AVC bias is placed on the grids in paragraph 81 with 
even a weak signal. This may be avoided by using a special 
DAVC diode circuit which prevents the AVC circuit from work- 
ing until the AVC voltage exceeds a predetermined value. This 
circuit may have all of the troubles indicated in paragraph 81 
and may also have a defective tube. Tests are similar to those 
set forth in paragraph 81. 

SECTION III 
POWER SUPPLY CIRCUITS 

83. GENERAL. Circuits illustrated in this section are typical 
and simple illustrations. Actual circuits in each particular in- 
stallation must be studied and tests modified to fit each individ- 
ual circumstance. 




-• FIELD 



LOW 
VOLT. 



w\ 



riCLO 

COIL ». 



VI 



/U 



RFC 



□ 



HIGH 
VOLT.I 



VOLTAGE REGULATOR 



Y/w///mm?w\\ww/////mmw////. t 



I 



1 ^g^j^p 



"S 




A. 



FIGURE 16. Generator circuit. 



84. GENERATOR. A typical generator circuit is shown in f igure f 
16. The armature is rotated by hand in a magnetic field and 
produces a low voltage output at socket terminals No. 1 and 
No. 2, and a high voltage output at socket terminals No. 3 and 
No. 4. Failure to operate may be caused by defects in any com^ 
ponent, in wiring and connections, or adjustment of the voltage^ 
regulator. Test operation by connecting a DC voltmeter of suit- 



32 



able range to measure the low voltage output while the generator 
is being operated under load. Proper operation is indicated by 
the meter reading the correct voltage and with no flickering of 
the meter pointer. If the meter reads too low or too high, adjust 
the "C" screw, which changes tension on the armature of the 
voltage regulator, until the correct voltage is obtained. If the 
meter flickers, check the air gap between the armature and the 
electromagnet of the regulator and adjust the "A" screw to give 
the proper gap. Then, adjust the "B" screw until the flicker dis- 
appears. Check the instructions accompanying the generator for 
correct voltage and air gap distances. 

85. INPUT CIRCUIT-VEHICULAR POWER SUPPLY. The in- 
put circuit of vehicular power supplies provides power from the 
vehicular electrical system to the input terminals of the dyna- 
motor or vibrator power supply unit. This circuit will contain 
some or all of the following components : storage battery, vehicle 
charging circuit, fuze, and filter circuit. Trouble may be caused 
by defects in any component or in wiring and connections. Test 
the voltage at the input terminals to the dynamotor or vibrator 
power supply unit. This must not exceed manufacturer's speci- 
fications. Inspect the storage battery connections for dirty or 
corroded terminals. Inspect fuze and check the entire circuit 
for continuity. 



"• FIELD 

COIL » 



LOW 
VOLTAGE 



w 



JN 



LOW 

VOLTAGE 

INPUT 



FIELD 
COIL », 



RFC 



VI 



HIGH 
VOLTAGE 



/U 



HIGH 

VOLTAGE 

OUTPUT 



FIGURE 17. Dynamotor circuit. 

86. DYNAMOTOR CIRCUIT. A typical dynamotor circuit is 
shown in figure 17. The armature is rotated in a self excited 
magnetic field by current from the input circuit. Failure to op- 
erate may be caused by defects in any component or in wiring 



33 



and connections. Test by connecting a DC voltmeter of suitable 
range to measure the output voltage while the dynamotor is be- 
ing operated under load. Fluctuating or low output may be due 
to brushes sticking in their holder, low brush spring tension, 
dirty commutator, high mica in commutator, commutator out of 
round, open circuit in the armature, or low input voltage. High 
output may be caused by high input voltage or grounded field 
windings. Noisy operation may be caused by loose mounting or 
by worn or dirty bearings caused by improper cleaning and 
lubrication. 

87. NONSYNCHRONOUS VIBRATOR CIRCUIT. A typical non- 
synchronous vibrator circuit is shown in figure 18. A vibrator 
unit interrupts the flow of DC from the vehicular storage bat- 
tery through the primary of the power transformer at a rate of 



HIGH 

VOLTAGE 

OUTPUT 




inu/ P0WER 

VOLTAGE TRANSFORMER 

INPUT 

FIGURE 18. Nonsynchronous vibrator circuit. 

approximately 60 cycles per second. The resulting voltage 
changes in the primary are stepped up so that high voltage AC is 
present at the terminals of the high voltage secondary and low 
voltage AC is present at terminals of the low voltage secondary. 
A rectifier system must be used to obtain DC voltage for radio 
equipment from the secondary. Resistors R x and R 2 prevent 
arcing at the vibrator points and may be replaced with suitable 



34 



condensers to achieve the same effect. Defective operation may 
be caused by defects in any component or wiring and connections. 
Test by measuring the high voltage output with an AC voltmeter 
of suitable range. Operation of the vibrator unit may be heard 
or felt by touching the shield can. Absence of output voltage may 
be caused by sticking or open vibrator points, low input voltage, 
or open, short, or grounded wiring of power transformer. Check 
with a continuity tester. Low output voltage may be caused by 
low input voltage or by a shorted transformer secondary. High 
output voltage may be caused by high input voltage or by a short- 
ed transformer primary. Sticking vibrator points are usually 
caused by low input voltage or by defective resistors or conden- 
sers across the vibrator points. 

POWER 
TRANSFORMER 



INPUT g 




FIGURE 19. Half-wave rectifier. 



88. RECTIFIER CIRCUITS. 

a. Half -wave rectifier. A half -wave vacuum tube rectifier cir- 
cuit is shown in figure 19. Test the DC output and the AC input, 
test tube and other components, test wiring and connections for 
continuity. If the AC voltage is within 15% of rated value, meas- 
ure the plate voltage and current under load. Low plate voltage 
and low plate current indicates a weak tube. Low plate voltage 

35 




POWER 
TRANSFORMER 



"1 OUTPUT 



FIGURE 20. Full-wave rectifier. 



TO FILTER +l , 



POWER 



• ruwtR 

I TRANSFORMER 



INPUT 




OUTPUT 
TO FILTER 



FIGURE 21. Full-wave bridge rectifier. 



36 



and hi S h plate current indicates a short "f^-J^^ 

an open circuit in the load. +:*:«- „:- 

b. Fun-wave rectifier A f j£«^ c^^cing 
cuit is shown in figure 20. Buffer '^f^^J^f^^^. 
at the input. Test as in £»graph 88. A short d ^ ^ 
ser may cause the vibrator to burn oui, <t»u » 

ed before replacing the vibrator unit. , 

c. Full-wave bridge rectifier circuit. A full-wave bridge-type 
copper oxide rectifier circuit is shown in figure 21. Test as in 
paragraph 88. pQWER 

TRANSFORMER 




_,, LOW VOLTAGE INPUT i + 

F1GURE 22. Synchronous vibrator circuit. 

89 SYNCHRONOUS VIBRATOR CIRCUIT. A synchronous vi- 
brator circuit is shown in figure 22. This circuit is similar to 
figure 18 except that two contacts have been added to the vibrator 
armature so that a separate rectifier circuit is not required to ob- 
tain DC output. Tests are the same as those mentioned in para- 

37 



3£e£ "^ that the ° Ut > Ut ^ »» measured 






with a DC 
FUSE 



INPUT 




FIGURE 23. Output circuit-vehicle power supply 



90. OUTPUT CIRCUIT.VEHICULAR POWER SUPPI IES So™ 

to n a Tr°cuTt sYmilf ??*" X™ 6UPPHeS de ^7h e fr 'outTu 
to a circuit similar to figure 23. The filter circuit is required to 

eliminate variations and to provide steady DC voltaL Troubled 
may be caused by defective components^ wiring of conations 
Test the output using a DC voltmeter of suitable iZTlS 
reading indicates trouble in a previous circuit 1 W rtaS 
may indicate a defect in a previous circuit or in the filteT network 
or bleeder resistor. Check these components for opens shirts 
.grounds, or changed values. ' snons ' 



38 



CHAPTER 5 
TESTING STAGES 

Paragraphs 

91-102 

Section I. Receiver stages ZZZO. • \®H\% 

II. Transmitter stages 111-112 

III. Alignment procedure 

SECTION I 
RECEIVER STAGES 

91. GENERAL. Superheterodyne amplitude 

a. Superheterodyne AM receivers Superftetero > J 

modulated (AM) receivers are made ; up -o ^^f^^ 
figure 24. The beat frequency oscillator is not required 
receiving modulated signals. 



ANTENNA 

w 



RF 

section 



IF 
SECTION 



AF. 
SECTION 



RF 
AMPLIFIER 



MIXER 



IF 
AMPLIFIER 



IF 

AMPLIFIER 



LOCAL 
OSCILLATOR 



- DETECTOR 



BEAT 

FREQUENCY 

OSCILLATOR 



AF 
AMPLIFIER 



ki 



SPEAKER 



FIGURE 21. Superheterodyne AM receiver block digram. 
<-■ b Superheterodyne FM receivers. Superheterodyne frequency 
moduMed (FM) receivers are identical tc .AM ^^enj.^ 
that the detector stage is replaced by a dis £»»**» f^ ™ d _ 
a limlter stage is added between the last IF s age and the dis 
criminator stage. Other differences * re ™ n0 J '" " a * Ure a " d arG 
indicated in the following paragraphs when necessary. 

c Tests In the following paragraphs tests are outlined to pro- 
vide an indication of troubles in a particular stage. Before test 
mg any receiver stage for trouble, the repairman must check the 
polefsupply to assure proper operation of that stage. Otter- 
wile m 1 dng conclusions may be drawn from the tests indi- 
cated Manufacturers specify voltages and currents for proper 

, • 39 



operation of each radio circn if tw i 

ed on all questionable receiver 2 f^ 3 Sh ° uld be check ' 

dication o? the part icu ar cir u T a " d v , Sh + ° U,d P rovid * an in- 
other than normal " WhlCh trouble exist s when 

^^oi^^^ o :^^^ s of individual 
are simple, typical toZ^nS ^l^fT™^ These 
understanding of test procedure ' d to pr ° vide a better 

ufacturers diagram rnltTZsZ^TZ mo^eft "^ 
each particular case. modified to suit 

92. RADIO FREQUENCY AMPLIFIER STAGE. 

a. General. The purpose of RF amplifier sH™, i. ♦„ ■ 
signal voltage, improve selectivity and to rJl, mCreaSe 

Normal operation is indicated tj ^faWlSfrf CT "T^ 

a 1 tL7 d nal throuehout its fr «fbS 

A typical diagram is indicated in figure 25. uisiortion. 




FIGURE 23. Tuned RF amplifier stage. 



b Operation (1) Connect a VTVM to measure negative recti- 
fied voltage at the limiter grid or at the ungrounded side of the 
detector load (point A of fig. 30 or fig. 31) with respect to 
ground or cathode. ^i^ci io 

(2) Connect a signal source producing an unmodulated 

sett^l^Z^ C ° nilenSCr (0l) St the «"** """ 

(4) If a negative voltage is obtained on the VTVM the stare 
is operating. ' b 

(5) Tune the signal source and the RF tuning condensers 
across the band and note operation at all frequencies. 

40 



,„ If the above proeed- , ^^^T^.tZ 
tive intermediate stage, connect an Kb sigi 

41) to the output of the RF ^™J£fotZ nmiter ir de- 

D of fig. 25) and repeat as above, measuring 

tector voltage of the signal tracer. Drovidin g a signal at 

c. Calibration. With the signal ^^eZ set ^ e.act- 
the high end of the band and th RF tunin ^ the RF trim . 
ly the correct dial ^^^Z^e on the VTVMIn 
mer condenser (C 2 ) to oDtam ma mounted in paral el 
b above. Note: T* ~^« XA done'at the 
with the tuning condenser. Calibration cond enser 
high end of the band because the. eapaa ty of the 

is nearest that of the trimmer at J-h.s point 

d. Oscillation. RF amplifiers may oscillate ana p 4 

in the receiver. Remove the ^^S^^iS^th a 
ure the grid voltage between A ^.^Y' 2 check across the en- 
vtvm Tf ypro the stage is not oscillating, urn*.* ^ 
I T e V bld If the grid circuit does not have a revive load, meas- 
ure grid voltage of the following stage. 

01 RADIO FREQUENCY OSCILLATOR STAGE. 
93. KAUiu ruc/veu^x oscillator in a receiver is 

a. General. The purpose o ft ^ F ^^^ hete rodyned 
to provide a locally gener^ ^correct difference fre- 
with the incoming signal to P/° duc * .™^ ice as „ reat in am . 
q uency. The local signa =t be «£*«™ J £^ in ^ 
plitude as the incoming signal, a iyp^ 
ure 26. 




TO GRI D OF 
NEXT STAGE 



FIGURE 26. Shunt fed, Hartley oscillator stage. 



41 



c Alignment. Oscillator circuits must be aligned so tW tha 

ZTtt "T? f T UGnCy " Pr ° duced whe " thelocally fener 
ated signal is heterodyned with the incoming signal AlLnment • 
may be accomplished by adjusting a padder condens r fcTS 
26) in series with the tunimr conrlpn^Pr m. o *-* 2 ? g * 

abgnment 13 made at the high end of the frequency Ed H^ 

E$E£££E£r lh,s ~"< <* - ~ 

or fVu Af - ter , calibratin e ^e RF amplifier stage (par 92c) 
(2) Adjust the padder (trimmer) condenser to obtain max- 

d. Tracking. The correct difference fremiPnrv »,„«,* v. 
duced throughout the band. Check ft olZZuZVl^Tf 
the RF amplifier* and RF oscillator do not <4ack» (change 
identical amount as the gang control is tuned) readjust b£h ™ 
Plifier and oscillator alignment and calibration 

94. MIXER STAGE. 

•°' ? e "!u a !'u T I ie i " comin e s'wl is heterodyned or electrically 
mixed with the locally generated signal in the mixer stage S 
produces an intermediate frequency, equal to the diffe?ence be 
tween the original signal and the locally generated signal which 
retains all frequency and amplitude variations contaLd in he 
original signal. A typical circuit is shown in figure 27. 

b. Tests. The input of the mixer stage must "track" th P TIP 
amplifiers and the RF oscillator. The output stage must be 




TO PLATE OFl 

OSCILLATOR* 

STAGE 

FIGURE 27. Hwode mixer stage. 

. . ^;„tP freauency. Tests for operation, 
tuned to the correct mtemedwte toque ncy .^ The 

forms the functions of both mixer ana 

*?%£*£■ rf* * thMe for ° sci " a " >rs and mtors 

(par. 93 and 94). 



PRE- §io 
CEDING Q O 




OSCILLATOR , 
ANODE 



TO GRID OF 

NtXrSTAGt 



FIGURE 28. Pentagrid converter stage. 



a. General. The purpose of the jf^^Voperation at a 
as for the RF ampli fie r stages ^ lo v ver due ^ fa fl 

lower and constant toquency, use ot : tu P fa ^^ 

circuits, and optimum coupHng bet^ een rt a* . J by ^ 

than that of ** ^^ J£^bility tuned transformers, 
of trimmer condensers or un 

43 



The latter have iron cores (nf or^ • i 

be adjured to chan 8 eT ,nd„e,a n ~£TT"} ^ ™ y 
mcs. All couplinc i„ ip .».„.?? / transformer wind- 
the IF s ta E e s must oe broad^noU ¥ "' ansformcr - Tuning of 
Ancles. A , ypi ca, ^M^S^^T ""and of 

"a — \. >~ — — 




At 



IT? T 



FIGURE 29. IF amplifier .Use.' 

^^^ simiiar * th - 

mixer i„ put (polnt B ^ ^SS «^ ^ « be W to the 
should be disabled. To check the wJ 5£ RF 0SclIlat °r 
reading at the correct IF ^ «. ba " d ™, dth ' n °t* the meter 
stol source a 11^ , ™ « the f/equency of the 
side of the correct I^and re pe at L* manufactu ™ °" each 
should be approximatel/tlfe ZTJll t^Z^lF. *""*» 
97. LIMITER STAGE. 

«5uKo2S ln m f?e r q u S enS modu^ '"J""*" ° f a " IF 
amplitude variations ^SS"^ ™™> t0 e,im ^e 
by operating the stage with low lull a S 1S accom Plished 
and with grid leak bias so tw w &nd SCreen * rid stages 

tained with relatfvely'ow input v^ '"^ ^^^ ,B ° b " 
dicated in figure 30 P ^ A typicaI circu * ™ fa- 

cee b d a^onowsf 10 "' T ° ** ^ aCtion ° f the *"«« stage, pro- 

Parag^t £ £2 £ X'oSat "Sf T* " * 
the IF signal should be very low SC1 " at ° r - The st ™£th of 

(2) Connect a VTVM to mpatsi,™ r>r> i- •* 

(point A of fig. 30 and ground) ° '""^ gdd V0,ta ^ e 

(3) Connect another VTVM to measure one half of «.. a- 
cnminator output (point E of fig. 32 and ground) dlS " 

44 



(4) Increase the strength of the IF signal gradually and 
note the meter readings. Both readings should «*^ «™£ 
ually until limiting action starts. Then the limiter gn voltage 
should continue to increase while one half the discriminator out- 
put remains relatively constant. _ . 

(5) The limiter grid voltage at which limiting action starts 
is specified by the manufacturer. 

c. Alignment. The input circuit is aligned in the same manner 
as the IF stages. The output circuit is aligned at the same time 
as the discriminator input, and a VTVM is connected to one half 
the discriminator output for an indication of proper alignment 
(see par. 99). 



DISCRIMINATOR 
STAGE 




A-f 7 



B* 



FIGURE 30. Limiter stage. 



98. DETECTOR STAGE, 

a. General. The purpose of the detector stage is to convert var- 
iations in amplitude of the incoming signal into audio frequen- 
cies. Superheterodyne amplitude modulated receivers usually 
employ a diode circuit similar to figure 31. 

b. Operation. To test for operation of the detector circuit, feed 
an amplitude modulated signal into the receiver antenna circuit 
and connect headphones across the detector load (point A of 
fig. 31 and ground). The signal should be heard. If a modulated 
signal is not available, feed the correct unmodulated IF signal 
into the mixer input as in paragraph 96, connect a VTVM to 
measure negative DC voltage across the detector load (point A 
of fig. 31 and ground), and note the voltage developed. 

c. Alignment. If a tuned circuit is included as in figure 31, 
align by using an unmodulated signal as in b above and by ad-, 
justing the trimmer condenser or coil to obtain maximum meter 
reading. 

45 




-<' — wwwv 



\[ 



JO GRID OF 
AF AMPLIFIER 



FIGURE 31. Diode detector stage. 



99. DISCRIMINATOR STAGE. 

a. General. The purpose of the discriminator stage is to convert 
variations in frequency of the incoming signal into audio fre- 
quencies. A typical circuit is shown in figure 32. 




FIGURE 32. Double diode, differential discriminator stage. 

b. Operation. Operation of the discriminator circuit may be 
checked by feeding a correct IF signal into the mixer input as 
in paragraph 96 and noting the presence of a negative voltage 
across one half of the discriminator load (point E of fig. 32 and 
ground) with a VTVM. 

46 



c. Alignment. The discriminator should produce maximum 
neUve'voltage in b above when the mean frequency „ app bed 
and should produce zero voltage measured across the ^t re load 
(point D of fig. 32 and ground). To align the discriminator pro 

ceed as follows: rtW _ f j„ nr vnlHc-p 

(1) Connect a VTVM to measure negative DC voltage 

across one half the discriminator load (point E of fig. 32 and 

gr ° Un ( 2)* Feed a correct mean IF signal into the mixer input as 

" ^rTu-L discriminator primary (limiter plate circuit) 

t0 ^I^Z^nt indicate false zero as in para- 
graph 22c and connect to measure DC voltage across the en tl re 
discriminator output (point D of fig. 32 and gr ou M) 

(5) Tune the discriminator secondary to obtain zero volt- 

agG * (6) Shift the frequency of the signal fed to the mixer equal 
amount in each direction from the mean frequency and note 
that the reading of the VTVM increases and decreases equal 
amoun s. This indicates that conversion of frequency variations 
fnto audio frequencies can be accomplished without distortion. 

(7) If meter readings in (6) above do not change equal 
amounts, realign the discriminator by trial and error. That is 
adjust the primary slightly different from its .first setting and 
readjust the secondary, then repeat (6) and (7) above until the 
discriminator is properly adjusted. 



TO 

DETECTOR 

LOAD 




A+ B+ 

FIGURE 33. AF amplifier staee. 

100. AUDIO FREQUENCY AMPLIFIER STAGE. 

a General AF voltage amplifier stages are used to increase 
the audio signal voltage sufficiently to drive a power amplifier. 

47 



to drive 
operate 

in push- 
A typi- 

by feed- 
. 33 and 
antenna 



The power amplifier stage produces the power required 
a loudspeaker or headphones. All voltage ampE 
class A, and power amplifiers operate class A, or class B 
pu ] Audio amplifiers must amplify without dTstortfon 
cal circuit is shown in figure 33. 

b. Tests Operation of AF amplifiers may be checked 
mg an audio signal into the input circuit (point B of lig 
ground) or by feeding a modulated RF s gnal into the 
input and listening at the output with headphones! 

101 HEAT FREQUENCY OSCILLATOR STAGE. For the re- 

ception of CW signals a BFO stage must be provided This ta g " 

Wiethe ?F ^ 7 r 1 Wh ! Ch heterod ^ in the detector afe 
with the IF signal to produce a beat note in the audio range. 
When reception of modulated signals is desired, the BFO must 

oJ 11 ! 1' r GStmg 1S ldentlCaI t0 that of the RF os "Hator ex- 
cept that the frequency will be the same as the intermediate fre- 
quency. Alignment is accomplished by zero-beating to an in- 
coming signal of the correct IF. 

102. DC AMPLIFIER STAGE. 

a. General A DC amplifier stage may be used to amplify any 
DC voltage. A typical stage is shown in figure 34. Note that the 
polarity of the voltage is reversed. That is, a positive voltage 
applied at the input will cause the output voltage to become more 
negative. • 




rilTER A + 

FIGURE 31. DC amplifier stage. 



48 



b. Tests. To test a DC amplifier stage, measure the voltage 
change at point A of figure 34 with respect t ground, and the 
voltage change at point D of figure 34 with respect to ground. 

SECTION II 

TRANSMITTER STAGES 

103. GENERAL. 

a. CW transmitter. Transmitters for CW telegraphy are made 
up of stages as indicated in figure 35. The number of amplifiers 
may vary according to the requirements of the system. 

b. AM transmitter. Transmitters for amplitude modulated 
radio-telephony are identical to CW transmitters except that an 
audio section is added, and an RF amplifier stage is used to mod- 
ulate the carrier. 

c. FM transmitter. Transmitters for frequency modulated 
radio-telephony are identical to AM transmitters except that 
modulation of the carrier is accomplished by a reactance modu- 
lator stage or in a special non-linear coil, and one or more RF 
amplifiers are operated as frequency multipliers. 

V 



OSCILLATOR 



BUFFER 
AMPLIFIER 



POWER 
AMPLIFIER 



FIGURE 35. CW transmitter block diagram. 

d. Testing. In the following paragraphs, tests are outlined to 
•<- provide an indication of troubles in a particular stage. Before 

testing any transmitter stage for trouble, the repairman must 
check the power supply to prevent misleading conclusions when 
power supplies are defective. Extreme care must be exercised 
to avoid contact with high voltage in transmitters. When opera- 
tion of a transmitter stage is doubtful, all voltages on tube ele- 
ments should be checked and compared with manufacturers 
specifications. 

e. Schematic diagrams. Schematic diagrams of typical indivi- 
dual stages of a transmitter are shown in succeeding paragraphs. 
When testing radios, the manufacturer's diagrams must be con- 
sulted. 

49 



104. OSCILLATOR STAGE. 

a. General. The oscillator stage generates a carrier wave whose 
frequency ,s determined by the constants of the circuit Normal 
operate is indicated by the ability of stage to product a S 

tl fn,,n reCt / re<1Ue ^ Cy and ° f SUfficient am P ]it ^e to excite 
the following stage. The circuit may be similar to that of figure 
26 or may be crystal controlled as in figure 36. 

(StXnsr c ,r ect / vtvm at the grid ° f the ° sciiiat ° r 

(point A of fig. 36) and measure the negative rectified voltage 
with respect to ground or cathode. Zero voltage indicates an in- 
operative oscillator. The amount of rectified voltage indicates 
the relative activity of the crystal or amplitude of the oscilla- 
tions. Consult the manufacturer's specification to determine if 




FIGURE 36. Crystal oscillator stage. 

this is sufficient to excite the next stage properly, or compare 
results with the voltage obtained with same circuit in a trans- 
mitter known to be good. Check the operation throughout the 
frequency band of the oscillator. Operation may also be checked 
by tuning a good receiver to the oscillator frequency and notimr 
the presence of a CW signal. 

c. Calibration. If the oscillator is crystal controlled, the fre- 
quency is fixed by the crystal used and cannot be changed If 
the oscillator is not crystal controlled, calibration is accomplished 
by zero-beating to a frequency meter or to a calibrated receiver. 
This may be done without electrical connections if the frequency 
meter or receiver is in close proximity to the transmitter. Set 
the oscillator dial at a frequency near the low end of the band and 
set the calibrated meter or receiver at the same frequency. Ad- 
just the oscillator padder condenser (C 2 of fig. 26) until' zero- 
beat is obtained. If a trimmer condenser is used, calibrate at 
the high end of the band. 

50 



10 1 ! RTTFFER AMPLIFIER STAGE. 

10 :: ™x*. ,-,-. - •£ as rss £ 

ceiver to the output '^rrniatortoperltU alone and when 
of RF. 




„ «„ n ff . « m «lifipr Rtaee with plate neutralization 
FIGURE 37. Buffer ampmier siage wim F 



e Tuning. Tank circuits of buffer amplifiers must be tuned 

to "esonance a. the desired frequency, ^"^fiffift 

the crid and plate tuning condensers (C, and C, of fig. «) to 

obtata maxhnum rectified grid voltage and mm.mum plate cur- 

ent « S- antenna current. If ^a* meters ° re "<* 

"^ ^tlTTvtrme^rn^^tified grid 
V0,t T2, <P Ex n citf thfsCbyt'SS'ihe oscillator at desired 
freq °(SrAdju»t the grid tuning condenser for maximum meter 
readi (4) Connect the VTVM to measure negative rectified grid 
V ° ta T5)°TdIusf thl'pfatc'rmng condenser for maximum motor 

reading. , . ; , 

a m- ™< Tf tranir tuning is used, the buffer tank circuits 
must"'*- K" other circuit, Proceed as **«: 

51 



(1) Connect a VTVM to measure negative rectified trnd 
voltage (point A and ground of fig 37) rectified grid 

a S e o^S< ™° ™ eaSUre ne£a " TC "*"- «* "«■ 

(5) Adjust the plate circuit trimmer condenser for maxi 
mum meter reading. wnuenber ior maxi- 

(6) If padder condensers or permeability tuned coils are 
used for aligning, continue. 

(7) Excite the stage by operating the oscillator at a fre- 
quency near the low end of the band. 

(8) Repeat (3), (4), and (5) above adjusting padder con- 
densers or coil cores for maximum meter reading. 

(9) Repeat the entire process until no further change in 
settings is required. 

e. Neutralization. Due to the grid to plate capacitance of 
vacuum tubes, transmitter amplifiers may oscillate. To prevent 
oscillation, a system of neutralization is provided. Figure 37 
shows a plate neutralization system in which a voltage is fed 
back from a balanced plate tank circuit to the grid circuit through 
a neutralizing condenser C„ so that it cancels the regeneration 
due to interelectrode capacitance. Grid neutralization is similar 
except that the voltage is obtained from a balanced grid tank 
circuit and fed to the plate circuit of the tube. To check the neu- 
tralization of an amplifier proceed as follows: 

(1) Remove the plate voltage from the amplifier, but be 
sure that the filament is lighted. 

(2) Feed an exciting signal from the previous stage into 
the grid circuit. 

(3) Tune the plate tank circuit through resonance and note 
the presence or absence of R.F in the plate tank at the resonant 
frequency This will be accompanied by a dip in rectified grid 
SS ^0 ? heck ' measure the rectified grid voltage with a 
VTVM and observe dip, or touch a neon bulb to the plate tank 
or couple a flashlight bulb and loop of wire inductively to the 
low voltage end of the plate tank coil and observe the lamp light 
If the amplifier is properly neutralized there should be no dip in 
rectified grid voltage, or no glow in the neon bulb or flashlight 



52 



(4) If (3) above indicates that the amplifier requires 
neutralization, continue. A ^ eM . with a screwdriver 

(5, Adjust the «^ "SE^S^ST^.. 
made of insulating material until the test inuu, 

is neutralized. ♦„#««, neutralizing condenser was 

(6) If an adjustment ^/ h /.^™ "f the neutralized 

rlrrntt Hias is always considerably past cut-otl. virtually any 
circuit. Bias is away nr>pr ated as a frequency multiplier. 

RF amplifier circuit may be operated i as a «j ' 
A special type of multiplier stage is shown in figure 38. 




FIGURE 38. Push-push frequency multiplier circuit. 

b Tests. Tests and adjustments for frequency mutiplier stages 
are identical to those for buffer amplifiers except that in order 
to assure tuning the plate circuit to the correct «, t 
input a properly calibrated frequency meter must be used to 
check output'of "he multiplier. Neutralization of frequency mul- 
tipliers is not required. 

107. POWER AMPLIFIER STAGE. 

a General. Power is furnished to the antenna by a power am- 
plifier stage. This stage is similar to other amplifiers except 
tha it is de igned to provide large quantities of power If bias 
s prov ded by a gridLk, the stage must not be operated with 
ow excSon for this will allow very high plate current to flow 
SSS'Sor damage the tube. A typical circuit is ,„di- 
cated in figure 39. 

53 



OUTPUT 




FIGURE 39. I«ush-pull power amplifier stage 



b. Tests. Tests and adjustments for power amplifier stares 
are identic, to those for buffer amplifiers except that, since Se 
power amplifier stage is coupled to the antenna, it i very im! 

Z7do no^ i^nV 11 thG antenna SyStGm ° r in antenTcou- 
phng do not detune the power amplifier plate circuit. The power 

amplifier plate current should be carefully checked to avoid ex- 
cessive current. When properly tuned, the plate current will be 
a minimum. In adjusting push-pull neutralization, the two con- 
densers should be adjusted together and their capacities should 
be kept equal to prevent unbalance and undesired coupling. 

108. AUDIO SECTION. When the transmitter is amplitude or 
frequency modulated, an audio section consisting of one or more 
audio amplifiers may be used. Tests are the same as for the audio 
section of a receiver. 

109 AMPLITUDE MODULATION. 

a. General. Amplitude modulation may be accomplished in any 
RF amplifier stage The audio voltage may be impressed on 
the plate, grid, or cathode to provide amplitude variations , in the 
carrier wave. All amplifier stages after the carrier has been 
modulated must be class A or class B in push-pull. The modu- 
lated stage should be operated class C and excited to saturation. 

b. Test. The most effective test of the modulated stage is meas- 
urement of DC plate current with a milliammeter. The average 
DC plate current should be the same both with and without mod- 
54 



ulation. With plate modulation, if **£**££%£*£ 
modulated, possible troubles are ,: i^fficjmt ^^ 
sufficient grid bias or amph ' -^ d. ^ P ^ 

rent increases, possible troubles are. au * parasitic 

complete ^^s^i^ot^e^^^^^^ . f 
oscillation in the modulated ^ph^er J ed probable troubles 
the plate current '^^^S^L^o section, or 
are: too much RF excitation, cu . If th late cur . 

amplifier plate circuit not loade d«Jta« ^ are : P excessive 
rent increases when modulated possible r 
audio voltage, distortion in the audio section, gn 
or incomplete neutralization. 

-:: =zr=» ~ *— it 

phase of the carrier wave is modulated. 

j t *«». a fvniml reactance modulator circuit 
h Reactance modulator. A typical redtwii^ 

isL^ated in figure 40. The oscillator must not be crystal con- 
trolled. 




ro 

IF 

FIGURE 40. Reactance modulator stage. 

c. Nonlinear coil. A typical nonlinear coil circuit is indicated 
in figure 41. The oscillator may be crystal controlled. 

d. Test, If t^^^^S^^ 1 ^ EM 
ctcST— noting reception on 
a good receiver. If the carrier drifts, the probable cause of th 
nonlinear modulation is improper grid bias on the reactance 

S3 




-lb 



PM OUTPUT 
TO RECTIFIER 



FIGUHE 41. Nonlinear coil circuit. 



modulator stage. Amplitude modulation in the transmitter out- 
put may be detected by observing the antenna current. It should 
not change when the carrier is modulated. The probable cause 

ats u s tgT: du,ation is improper tuni - - cou P Co?th s : 



SECTION III 
ALIGNMENT PROCEDURE 

111. RECEIVER. 

a. AM receiver. The following instructions may be used as a 
AM?ece*erI ** ** ""*"* ^^ ° f ^or^oTynl 
.1 f (1 > C0n " 6Ct an out P«t meter across the voice coil or to the 

££l fa to I P f r and gr0Und ' ° r Cath ° de ' if a »«M*«S 
signal is to be used. Connect a VTVM to the AVC lead and 

56 



ground, or cathode, if an unmodulated signal is to be used. If 

denser or by removing the crystal. _ couDle to 

(3) Adjust a signal source to the correct IF and couple 

the mixer input. section for maximum 

(4) Align all tuned cxrcmt s of the if eet , ^^ 

meter reading, starting at the deteexor «ti brought 

Reduce the output of the signal source as each stage brough 
into alignment. Keep the set volume control on full 

*° d TS^.E~ ^ "-"- trta - 

mers for maximum meter reading. ( ,, 

(7) If the RF oscillator uses a padder condenser tune he 

(7) II tne *r u d and adjust the 

set and signal source to the low ena oi i «R OC k" the dial 

padder condenser for maximum meter reading. Rock the 
slightly while making this adjustment. . 

(8) Tune the set and signal source to the high end of the 
band/and readjust the RF oscillator trimmer. Restore the AVC 
if previously disabled. . 

(9) If a BFO is used, turn it ON and zero beat the receiver 
by adjusting the BFO trimmer or padder. 

b. FM receiver. The above instructions are apphcabk|oFM 

receivers except thai *»£%«^^£FJ^^ 
must be connected at the limiter gria «ui *» .• ,. 

Inlt^^^ 

i accomplish the discriminator alignment as indicated m para- 
graph 99? Check band width of all IF stages as indicated in 
^ paragraph 97b. 
' c Precautions. After aligning, check the operation of the re- 
ce ^er o^rthe entire band. Oscillation may occur ;n the IF or 
RF stages generally resulting in squeals as the set is aligned or 
funed Probable causes are poor ground connections at the con- 
tuned. Frobawe «* condensers in cathode, screen 

g^or p a S^S^he bypass condensers by shunting 
grid, or piate cirvu imnroner valve of screen grid volt- 

with a good condenser : il ^ n on im S k the resistance of the screen 
age may a so cause <^^^^ e a tton «" the IF section 
ZLZZ£Z SLaTSSe. £ independent of dial tuning. 






58 



CHAPTER 6 
TESTING RECEIVER SYSTEMS 

Section I. Voltage and resistance measurements ■■■■•;;;;;;;;;;;;;;;;;;;;; jgzj| 

II. Signal tracing ■■■■■■■:■"■;- 131-iaj 

III. Other operational tests 

SECTION! 
VOLTAGE AND RESISTANCE; MEASUREMENTS 

113. GENERAL. ,. rGCe i ve rs 

, ^reduction. Volta. and ^^fi^£* 
provide the most accurate and ■ «'«*™£ defective sta ge or 
ble to a particular stage ,or arcurt -^ "component or adjust- 
circuit has been identified, the wdiviaua i f prev ious 

ment at fault may be located ^^^ isolat P ing d e- 
chapters. Quicker but less elective i Voltage 

■fective stages are outlined ^£?^£ the'mlnufal 

r' ^jTi'SiSSnf^Sr^- normal. All voltage 
turers data to determine v . operating but receiv- 

measurements are made with the receive 
intr no signal unless indicated otherwise, 
b. Comity test, Where continuity tests fo^ ^ componen s 

are suggested, the resistor "g^^^Mni 
doubt should be disconnected from the circu it spection f 

its resistance or testing for *^ f "^ Sfnot be 
the circuit reveals that the component in question * 
Erected b y other circuits to w^-jS^^Sd^SX 
ways remove power supply voltage belore m b 

"tests with an ohmmeter. wino- inv set 

, , » j„ ( „ witVi this method of testing, any sci 

are not absolutely necessary It £P£^ g|miIar set that is 
by measuring voltages and re ^™ CS yalue of certaln items 
not defective. ^^.^^^ t facing, and tube data 
such as condens f . s . a " d /Xt to expect when testing. For in- 
ST« thfh^tttal £ Opiate of an amplifier tube 

' eft 



the vollase drop through thfresistence L"""" "3^ d ° e '° 

way or that the resistor was open " ^ Sh ° rted in S ° me 
d Power supply. Satisfied that the tubes are not defective 

sTort drcuits at Jl e J ing the Pr ° P6r V0,ta * e or whether 
and caus7n* t t £? T* fJ* overloadi ^ the power supply 
and caus ng it todehver less than the proper voltage. Likewise 

cau: e The m vo a ita?e a? ^ G ^ t0 * «**> ?n the radio ^ S 
cause the voltage at the power supply output to be higher than 

normal because the load on the power supply had been reduced" 
114. VACUUM TUBE STAGES. 

fig a urI1 t 2 aj f e o r m ^';T entS - In genera1 ' the V0ltages indicat *d in 

Oblus Ln[f' P T 6 V Tu m tUbe StageS ma ^ be measured. 
Obvious modifications will be necessary for other types Fewer 

measurements than indicated may be required to bcate the de-. 




FIGURE 42. Meter connections for checking pentode stage. 

b Abnormal indications. Some abnormal indications and pos- 
sible causes are listed below. P 

formlr\St filament ri ta ° e (E ' h Burned out tube, trans- 
former shorted or grounded, or dropping resistor shorted. 

60 



(2) Low filament voltage. Connecting wires shorted or 
grounded, or transformer shorted lor ^^ dropping re - 

(3) Zero filament voltage. Open transform 

sistor, filter choke, or series tube filamen • ^^ 

(4) High cathode voltage (E k ). Open mas 

plate voltage, or open grid circuit. ltaBe low sc reen grid 

(5) Low cathode voltage. Low plate voltage, io 

VOlt T6)°LrcaLl voltage. Shorted WJ^S. 0r 
screen grid condenser, no plate ****£«%$& Snser. 

(7) Positive grid ^eJEJ. Leaky ' * J J Qr high 

(8) High negative grid voltage, rtign pi 

plate current. T n i n tp r screen volt- 

(9) Low nejjatfre grid voltage. Low plate 

age, low plate current, ^%^ correct) . Open grid circuit, 

(10) Zero grid voltage W *«f^.{ or ^ n0 p i ate voltage, 
shorted cathode condenser open ^ate c.rcmt or P 

(11) Hfoft plate voltage (E P ). WeaK tuue, u 

circuit in other stages. c horte d plate circuit or screen 

(12) Low plate voltage, bhortea pwic 

grid condenser. id circuit shorted 

n^ ffiafe p^s current (i P ;. upen *> llu 
U«>; « i y«- > y „„ „„ clmrted coupling condenser. 

^ 14 Xr;r :r;*Cn P?ate circuit. or .routed 

P ' a,e (l»^ Vlate current. Open S creen erid circuit, or low 
plate voltage. 

115. TEST PROCEDURE. A typical circuit diagram , fo^ an AM 
superheterodyne receiver is shown ^^^f^t. 



must be made 

116. POWER SUPPLY. 



a Power supply normal. The entire power supply may be 

tested by Lrsurlng the DC voltage between test pomta 4 «d 1. 

If normal, proceed to test the audio output stage (par 117) 

b Power supply voltage zero at test point 4. Remove the recti- 

b. lower suppiy u j, between test po nts 2 and 3, 

fier tube. Measure the , AC oltage *£™ J ^ T1 

2 and 1, and 3 and 1. If '"£&£ ^'age is normal at test 
is defective or -^^f^™£; 4 ^4 or C45 may be shorted, 
points 2 and 3 and zero at test poini 4, 1^* u 












ll 


I " 


■ p' 




A~L 


CC 




/w\ 


u 




pi 




u. 
o 

UJ 

a. 








10 








a 




© 


© 




U 


<suu 


UjUSJLfiAiL 


,oc 


* 


nmwws^ — 


-•< K 




1 , 


*£ 
.!?* 



a 

9 

CO 

•"* 

H 
OS 

5 



62 



L14 may be open or shorted to ^L\^^^ in the 
high voltage leads of component^ Shorted to ground. Use an 
rectifier tube winding of 11 may determ i ne which stage 13 

ohmmeter to test these components. ^ reconnect 

shorted, remove the- high Yf ^ e ] s found'which causes the DC 
them one at a time until tn \ le f°- normal When the defective 
voltage at point 4 to drop ^ow n^J ^ rf trQU . 
stage is isolated, it can be tested to determ 

ble - A t ., 4 low Follow the method outlined 

. c. DC voltage at test pom 4 £*• F ^ ^^ ^ volt . 
•above. In this case, if C44 is open 
a g e at test po,„t 4 w,„ ,» £- - ^ ^^ ,„. 

d. DC voltage at test point 4 abote r. gd 
dicates a reduced load on 1 the pow r upp^pr^ ^ ^ 
an open in some stage. Check : aUtu t Then 

stage is found where normal voltages a ^ ^ ^^ 

check all components betweer the tube of compo . 

supply for open circuits. Check o\er 
nents and associated wiring. 

„™ c-TArp Pi mi re 43 shows a push-pull 

117. AUDIO OUTPUT STAGE. F ^^ ]ement DC volt ages 
audio output stage. Measurements of tube ^elem 

should be made first, especially at ^tV™*™^ • voU . 
Then, proceed to the filament cireu* ^^J^sl R27, R28, 
age at each tube. T2 ' L1 ^Y5 c wked for opens, shorts, and 
R30, C38, and C3 shou, be ^h eked fo^p ^ ^ 

^cXtf sS"^ sU and cause a 

heavy overload. 

-- lMnMPI P R vN n PHASE INVERTER. Measure 

118. AUDIO AMPLIFIER AND N« 

tube element voltages especially at test pomt 5^, 

C36, R31, C40, R26, R29, C37, R24 R25 .and ceH ^^ 

values, shorts, or open circuits. For ™f" ce '£ J\ nt B> 

or R26 open, there would be no voltage at test point 

119. SECOND DETECTOR Check — ^-of^l. 

L10, L12, L13, RIO RH. « - ^ C34 - g* ^ ^ 

voltage of diode tube should be checkea i, 

be tested because they couple Lll to LJ. 



puints / and 8. Check continuity and value of T n t?k pt 
cate R. .^."TcS^SS: ^ <teS ' P< " nt 8) WM " d M - 

H'SlSt 1 *.**" V0 " a,res ' cs P« M y »' test 

^^v u;rrtte t ,t e po i » p tto t tV nd,cates a - ° pca 

25 »r £ m r RF •^MS.'ESSS -: 

troi tube may be measurer! Tf it ^;^„ * ^ l ' ton 

C18 and P21 fr,r t T i « d ffers from nor mal, test C30, 

a^oiT/f H' M T re \ ube e,ement vo,tages > "P^ly 

at test point 12. If the voltage at test point 10 was normal the 
same voltage should appear at the screen grid terminal of the RF 

C5 P C4 'r U rV 0Ck f nw Ck r SiStanCG ° f R2 and R1 ' and check 

Iht t AV i C CIRCUIT - The Set must be worki "S well enough to be 

at t^S £ IIS S1 ^ a, V A ,0W V ° Itage Sh ° U,d be P-- 
at test point 23 when the set is tuned to a signal. When no signal 

ChecT?!^ V s e h d ;r? e ^ ^ M ^ «™ 
chants In values ""^ and R12 a " d RU for <*«» <* '"so 

NATOR T Secl T fn FRE ™™ CY CONTROL AND DISCRIML 

tnii u Check / 1 ament voIta ^ of both tubes and the tube ele- 

S&^Tis K e auto r tic . frequency contro1 tube > ^y 

at test point 13. If no voltage is found at test point 13, measure 

R9 R17 R « gG h 1 7' 1 teSt C46 f ° r Sh0rt Check values o? Rs! 
R9, R17, R18, and check C22, C23, and C24 for shorts 



126 FT FOTRON-RAY TUBE. Check tube element voltages Vol- 
l^b. JiLELlKUXN-iwvi iu «hould vary with signals 

age in the AVC circuit at tetpoi^ 23 ^ u^ V ^ ^^ 
and should correspond to the \oltage ai i, 
ray tube. Check the value of R14. 

SECTION II 

SIGNAL TRACING 

127. GENERAL. Signal tracing is the P^^ f r i° , {^ nS no a r ^; 
nal through a set and observing w ^^nt'X^e- 
behavior-that is, distorts fades, etc At t V ^^ 

parture from normal ,s observed eUJ 

ined and tested to determine the exact cause, 11 l 
method for signal tracing, al test points and part names ^refer 
to the AM superheterodyne ^"^^^^"LSTto 
modifications must be made when appivmg 
other circuits. 

12 a ZZT£r ,oca ti „ 8 a troubte h, a deceive receiver, 

freauencv and some means 01 aeiecuni, mc i -* 

of the signal. A signal generator may be used to provide the s,g- 
nal or he signal may be "tuned in" from either a nearby or a 
dflnt transmitter. For checking the pjjjence an#tyo 
sijrnal either the defective receiver itself or a special sitnai 
tracer (par 38-41) may be used. The most appropriate tech- 
nfque wirTdepend on which of the alternate types of equipment 

iS b US Working from reproducer lo antenna. A signal of the proper 
frequency iJay be furnished by a signal generator-wither by a 
irequency may bridffinff ver from the appropriate stage 

commercial model or by oriafeiiii, y» „.„,n n .r nrrW Rv be- 

■ x of another receiver known to be in good «°^ ?™- ^J £ 
• • uv, tv,P nHtp of the AF stage next to the reprouucei 

ginning with the plate ot tne ^ back ^ 

and applying a signal to .each -.. tage ^^ 

wards the antenna, the detect »*f^ J t ^ ^ ^ 

m u 6 , the t 7*1 for itabU ty o , o^el the signal ; if, for example, 
added is tested for its aonuy iu i» ^ a , „ . 

the signal is introduced into the 8J.d «« 5U ' t r °/*^ '"f^ f r *t_ 
i iL i ; M +v,p rpnroducer distorts or fades out, me irou 

^bet^n the Anal and the reproducer have prevourt, 



been tested. 



c. Working from antenna to reproducer. If a s i ffna i generator 
W hich can furnish the proper frequencies is not Sable Z- 

SinTt t G h tU 7 d "'" 2™ ** Signal is RF ' " - "ec -sary fo 
begin at the antenna. Some special means of checking for the 

To^te th^ *"? mUS ? e USed ' SinCe the RF — b used 

Str J P T^""* of the de ^tive set directly. Instead, a 

signal tracer (either an untuned signal tracer or a receiver 

acrosT le!/ 00 ^ t mUSt bC USGd - The s * nal tr -er is conne ted 
troubl L h ' e ? ta * e V V ° rking awa ^ from th * antenna. The 
trouble has been located when the "tuned in" signal fades out 

SSS r8 h d i 8torted *w t ! ,e reproducer of the Signal tracer > si « ce 

all stages between that test point and the antenna have previ- 
ously been tested and found good. 

d. Precautions. Neither of the two procedures described below 

fZ e ^ reS u ! - that f Ve infalHble - Either furnishes indications 
for the checking of certain components. Only when the exact 

component at fault has been discovered, the trouble remedied, 
and the set put in working order again can the work of the re- 
pairman be considered finished. 

129. WORKING FROM REPRODUCER TO ANTENNA. 

a. Equipment. Assume that no commercial signal generator is 
available but another set of the same design as the defective set 
is at hand. 

b. Testing reproducer. 

(1) Turn both sets on and tune them to different fre- 
quencies. The good set should be tuned to pick up a modulated 
signal. 

(2) Connect one lead from the top side of the secondary of 
T2 on the defective set to the same point on the good set; connect 
a second lead from the other side of the secondary of T2 to the 
corresponding point on the good set. If the signal is satisfactory 
in the defective set, the reproducer of that set can be assumed 
to be good If not, check components of the circuit using methods 
in section I. 

(3) The general rule for this and for succeeding stages- if 
the signal is satisfactory, pass on to the next stage; if not, check 
the components of the circuit and make necessary repairs. 

c. Testing AF circuits. 

(1) Remove leads. 

(2) Connect one lead between the chassis of the two sets. 
This lead remains fixed throughout the remainder of the test. 
66 



(3) Connect ^^J^X**^ 
farad condenser) from test pornt 1 on th^ g ^ ^ 

point on the defectiv .set This br ^ . satisfact0 ry, the 

good set to the defective set. If the situ 

plate circuit of this tube is good. tube in 

(4) Check the T^^^£Z* tat point 19 
the same manner by moving tne secoiiu 

on the defective set to test point 21. PO ndenser to test 

(5) Shift the lead which »f?*« £X^gh ««s- 
point 18 on both sets. If the ^X^L^rlZ associated 
factorily, check components of the \ anou* 

with this tube. . , 1H t u p defective set 

(6) Change the lead <™™J^™Z^«£t»*o«' 
to test point 20. ^ f th % s ^ a "rTus circu" s assorted with this 
ly, check components of the various circu 

tUbe *m Shift the lead from test point 20 on defective set to 

(7) bnilt tne icctu „; rcu it of the phase inverter, 
test point 22. This checks the plate circuit oi in v 

including coupling to the output tube. defec tive set to 

grid and plate circuits of the AF amplifier 

«n cfcift the lead from test point 17 on tne ueieyuve d 

(9) Shift the leaa ir not satisfactory 

che* ZSt t^nl'det^an/audio staee and then 
£E aTa'ddMona, check on the audio stage ft*™* 

(10) Function..* of "»^^* e ^ c0 J e „iently when 
second detector stage can be checkea m 

^.e^ntettlf after al> other sta.es of the se, have been 

checked. * , rhprkinir of the AF stages 

nv\ With the exceptions noted, cnecKint ui ^ 

^ ^'IIVa. AF -JJ-^* K. Cached 

SS pS itonT^ ^^ the other end Iron, « 

point 17 on the defective set. 

d. Testing IF circuits microfarad 

(1) Attach the lead »hich V^^^Lh* the other end 

S£ 2£5£ c^^^dS^. Pa, on to the 



next operation only if the signal is satisfactory. Otherwise, check 
components of the circuit in detail and make repairs. 

(2) Shift the lead from Lll on the defective set to test 
point 16. This checks plate and grid circuits of the IF stage. 

(3) Change the lead from test point 16 on the defective set 
to test point 9. This checks the plate circuit of the mixer. 

(4) Checking of IF circuits has been completed. The IF 
signal from the good set is no longer needed, so remove the lead 
from test point 7 on the good set and from test point 9 on the 
defective set. 

e. Testing oscillator. 

(1) Tune in a signal on the good set. If the oscillator is 
tuned to a frequency above that of the incoming signal, tune in 
at the high end of the band; if below, tune at the low end. 

(2) Chassis of sets may be left connected. 

(3) Take the antenna lead from the good set and place it 
near the wiring of the oscillator of the defective set — to provide 
loose coupling. 

(4) Tune the defective set until a beat note is heard. This 
note should appear when the setting on the dial differs from 
the setting on the dial of the good set by an amount equal to the 
IF. (The setting on the defective set will be below when the os- 
cillator is tuned to a frequency above the incoming signal ; above, 
in the other case) . 

f. Testing RF circuits. 

(1) Tune the two receivers to the same frequency. 

(2) Connect the lead including a .001 microfarad condenser 
from test point 15 on the good set to test point 12 (plate of RF 
amplifier) on the defective set, thus providing an RF signal. In 
RF stages, signal tracing is less reliable than inr earlier stages. 
However, this connection gives a rough check of the control grid 
circuit of the mixer. 

(3) Shift the lead from test point 12 on the defective set to 
test point 14 (grid of RF amplifier). This gives a rough check of 
the plate circuit of the RF amplifier stage. 

(4) Remove the lead from test point 15 on the defective set 
and test point 14 on the good set. 

(5) Connect the antenna lead-in from the good set to the 
antenna binding post on the defective set. Signal volume or back- 
ground noise should grow stronger; if not, check the antenna cir- 
cuit. 

68 



* m l TL'XSrr'a^f automatic frequency con.ro. 
tubeaVncluded inTh^et are part of an automafc frequency con- 

tr0 ' 1'fL in a strong local signal with the automatic fre- 
(2) Tune in a strong he , em inop erat.ve. 

qU Ts)Ten r^i h and Sservette quality of the signal; 
if the sig^aUs still free from distortion the system . e.ther 

^rcfoi StS'trSr SttSE) ; some distor- 
tion h urn be evidLt, since the system is ■■*£*£. ' s <£ 
test reveals that the automatic frequency con rot ystem ^no 
operating normally, check components in deta.l and make nece, 



T Abb viated procedure. To determine quickly in what sec 
Hon ( AF,Tf:kF) "of the receiver the trouble is located, teste may 

1,6 "?» £er£r"t— d for the second deteetor (par. 
120b 1 ) If the signal comes through satisfactorily the repro- 
nucer and all audio'circuits aro good; if not, perform an steps 

in nrrW from the reproducer to that point. 

(2) Perform the test deseribed for the plate circuit of the 
mixer (par 129b(3)). If the signal comes through satisfac- 
torily, ill IF circuits are good; if not perfom tests in order as 
described in (paragraph 129d(l) and 129 d(2)) _ 

(3) Perform the test described for the antenna circuii 
(par 129f(5)" K the signal comes through satisfactorily the 
RF circuufand the oscillator are good ; if not, perform all tests 
in order as prescribed in paragraphs 129e and 129f. 

i Testing FM sets. The tests as described above are applicable 
to FM as tveli as AM sets. The modulated signal tuned ,n on 
the good set must, of course, be FM. 

130. WORKING FROM ANTENNA TO ^PKODUCER 

a. Equipment. Assume that no commercial s gnal generator is 
available, and no other set of the same or similar design. A 
signal tracer (par. 40) is at hand. 

b. Testing RF circuits. 

(1) Turn the defective set on. 

2 Make sure that the signal tracer ,s connected to its 
battels Connect the common lead to the chassis of the set. 

69 



(3) Unless a very powerful signal is available to be tuned 
in, or unless the sensitivity of the signal tracer is improved, ' 
connections to the antenna, or the grid, or plate of the RF ampli- 
fier will not yield a signal strong enough to operate the head- 
phones connected to the signal tracer. 

(4) Connect the RF probe of the signal tracer to test point 
16. Listen for a signal in the headphones of the signal tracer. 
The signal will probably be weak even in a normal set; if in- 
audible or distorted, check components of the circuits of the 
antenna, RF amplifier and mixer grid. 

c. Testing IF stages. 

(1) Move the RF probe to test point 9. The signal fed 
into the signal tracer is now IF, but since the tracer is untuned,, 
the connection with the RF probe will pick up this signal, too. 
The signal should be louder, because of amplification in the 
mixer stage. If not, or if the signal is distorted, check the 
tube and components of the plate circuit of the mixer and of the 
oscillator. 

(2) Move the RF probe to test point 16. The signal 
should be about the same level. If not normal, check com- 
ponents cf the grid circuit of the IF amplifier. 

(3) Move the RF probe to test point 7. Signal strength 
should increase. If the signal is not normal, check the tube 
and plate circuit of the IF amplifier. 

(4) Move the KF probe to the diode plate. Signal 
strength may drop slightly. If the signal is distorted or has 
faded out, check components of the detector stage. 

d. Testing AF stages. 

(1) For testing AF stages of the receiver, only the AF 
portion of the signal tracer is used. 

(2) Connect the AF probe to test point 17. If the signal 
distorts or has faded out, check components of the AF ampli- 
fier grid circuit. Note, that adjusting the volume control should 
affect signal strength at this point. The volume control may 
have to be set high to hear the signal. 

(3) Move the AF probe to test point 5. The signal should 
be stronger, due to amplification by the tube. If the signal is 
not normal, check the tube and plate circuit of this stage. 

(4) Move the AF probe to test point 22. The volume 
control may again need to be set high. If the signal is not nor- 
mal, check components of the grid circuit of this stage. 

70 



u ^ foQt noint 6. The signal should 

set, the signal strength ^^^.f^^ ci cdt of this stage. 
5. If not, check components of the grida 

(7) Move the AF g»* *£ PomtJO-^ ^J^ 
should appear stronger If not che ^ ^ ^ 
of the plate circuit. If the signal 

ponents of the plate circuit °\ th ?**™*° 2 J slgna l strength 

(8) Move the AF P" te ^ ^^f^, check compon- 
should be about the same as at point fa. 

ents of the grid circuit of this stage ^ 

(9) Move the AF probe . U ^ test point ^ 

should appear stronger. If not, cnet* 

of the plate eireuit of *'» "^ circull8 have u,,, 

e. Cheeking reprcKloeer. If the pre«d 6 ^ 

2t^C"S^Si^«S- set; if not, cheek c„ m - 

these circuits, see paragraphs 129g. 

* mTtLTf tages and the reproducer of FM sets can be 
check dulinglhe unluned signal tracer. To test the discnmi- 
nator conS the AF probe of the signal tracer to the un- 

^^^^^'T^^^r. 40) cannot, how 

o b u e tlin e ed1n £SS SI *" ^ fj^ff * ~ ™ 
set, follow the routine described in that paragraph. 

<- SECTION III 

OTHER OPERATIONAL TESTS 

mi nmFRAI Certain operational tests, such as circuit dis- 
131. GENERAL, certain i ntly b e used because of 

turbance and stage mufan* ^^J nce cannot be placed 
their time saving features Complete r ^ 

in these methods; .^f' ^^'^Wes rapidly. All 

t^Zr^^r^ rSer'to ft.' All superheterodyne cir r 



cuit shown in figure 43. Obvious modifications must be made 
when applying these methods to other circuits 
132. CIRCUIT DISTURBANCE. 

a. General. Circuit disturbance testing usually affords a quick 
method of locating the stage that is defective in a completely in- 
operative receiver. This test is based on the principle that a 
sudden change of current through the reproducer will result 
in a click being heard from the reproducer. 

b. AM testing. 

(1) Turn the set on, allowing sufficient time to warm up. 
Open the plate supply of either of the output tubes. A click 
should be heard in the reproducer. If a click is heard, the out- 
put stages, reproducer, their coupling medium, and power sup- 
ply are considered good. If no click is heard, the trouble lies 
in the tube or its circuits, or power supply, or in T2, L15, or 
L16. To check T2, L15, and L1G, turn the set off. Apply 4V4 
volts across the plate leads of the output tubes. If a click is 
heard, T2, LIB, and L16 are good. The trouble is in the power 
supply or in the B lead to the primary center tap of T2. If 
no click is heard, apply the 41/2 volts across L15 and L16. If 
no click is heard at this point, one of these two coils is defective. 
If the click is heard, transformer T2 is defective. 

(2) Open the plate supply of the audio amplifier tube. 
If this stage is operative, the plate current will go from some 
-value to zero instantly, causing a change in grid voltage in the 
output tube. This will result in a click being heard in the re- 
producer. The plate supply may be opened by pulling the tube 
from its socket. If this is not convenient, the same result may 
be obtained by momentarily shorting the grid bias. 

(3) To check the detector stage, apply 4V£ volts between 
ground and the plate of the second detector (plus to plate). Be 
sure the volume control is turned to near maximum volume 
position. A click should be heard if this stage is not defective. 

(4) The IF stages may be tested by the same method as 
used for the AF amplifier; or the grid may be touched by the 
finger to produce a click indicating the stage is good. In the 
process of producing clicks in the reproducer from stage to 
stage, when the point is reached where no click is heard, the 
trouble is generally between this point and the last point pro- 
ducing clicks. 

(5) For the mixer, oscillator, and RF stages, the plate 
current can be changed by touching their respective stator 
plates of the ganged tuning condenser. 

72 



(R) In case of zero grid bias in any stage to be tested it 
may be f und more 'expedient to pull the tube ^ »* "ocb*. 
or "to apply a low value of grid bias by ^^^^ 
between cathode and grid (minus J°.^! d >- . ^d tadrcSs 
result in a change of plate current if the ^ ™ d . ,to "™^_ 
are operating properly, thereby causing a ^ .^J^* 
tage on the following stage, ending with an audible ciick in 

the reproducer. . 

c. FM testing. Due to the leveling action of ^ hnnte- ^ ' 
circuit disturbance tests are not as precise with FM • re ^ vers : 
and certain modifications may be required. When a„ FMre 
ceiver is operating properly, with no ancomingr signal a «t«>ng 
hiss is heard in the reproducer. This hiss is due * the **on 
of numerous high gain amplifiers. If the hiss is strong 11 .a 
defective receiver, the trouble is probably in « n "'^ E * 
staro* and if the hiss is very weak, the trouble is probably in 
:Te^tirsta h ge S s. In FM sets it will ofter , bj > necessary 
to add external grid bias to cause a good click to > b heard 
through a properly operating stage. This may be done by con- 
necting the plus side of a battery (4V4 "«»» + US " a ^'T.fhll 
to the cathode, and the minus to the grid of a tube. A double 
click should be heard, one when the connection is made and 
one when the connection is broken. 

d. Precaution. Circuit disturbance tests are not always con- 
clusive, but in general lead quickly to fte defective stage in a 
totally inoperative set. After the defective stage has been 
found by fee circuit disturbance method, if the trouble „ ^not 
obvious, or can not be located through the principle of a jhai^e 
of plate current causing a click in the reproducer, another test 
should be used to find the exact source of trouble within _ the de- 
fective stage. This will usually be the method outlined m 
section I. 

133. STAGE MUTING. Stage muting is a rapid and usually ef- 
fective method of isolating hum and noise generated 1 within a 
set to the stage causing the trouble. To find the stage causing 
hum in figure 43 proceed as follows: 

a. Pull one of the output tubes from its socket. Short the grid 
to ground on the remaining tube. If the hum disappears, in- 
sert the other tube and pull the tested tube from its socke t. Re- 
peat the test of shorting grid to ground on the inserted tube. 
?f the hum continues when each tube is tested the power _supply 
is probably at fault. In all cases of push-pull output stages it 



will be found expedient to pull one tube from its socket before 
testing the other output tube. 

b. To test the AF amplifier tube, short the grid to ground. If 
the hum is stopped, proceed to test the detector tube. When a 
point is found that does not stop the hum, the source of this 
hum will be found between this point and the last tested point 
that did stop it. 

c. The detector stage may be tested by shorting the plate of 
the second detector tube to its cathode. 

d. IF, oscillator, mixer, and RF stages may be tested in the 
same manner as used for the AF amplifier stage. 

e. At very high and ultrahigh frequencies, and generally 
through FM receivers, an external grid bias will be found 
necessary to stop the action of a tube. The amount of voltage 
to apply between grid and cathode of a tube to stop its action 
will be determined by the characteristics of the tube and its 
circuits. 

f. Stage muting will be found convenient for locating the stage 
originating a hum or oscillation, but, unless the exact cause is 
obvious, the test method indicated in section I must be used in 
locating the defective point of the stage. 



74 



CHAPTER 7 
• TESTING TRANSMITTER SYSTEMS 

134. GENERAL. Voltage, current, and ^f^^Z^oi 
transmitters provide the most accurate and ^ ve ^™ t £ 
isolating trouble to a P^S^TnSX comment 
defective stage has been »*9£rf^* e methods outlined 

or adjustment at *™ lt J™J^%^£ measured with a 
in previous chapters. All voltages snuu , n 

high resistance DC voltmeter, P^^^Se L untes 
currents should be measured with a M»»»° „ 

otherwise indicated. Never work on a t™nsm.tter ™ ™ h 
voltages applied unless no other way is P°» s '«; * .™°™£" 
understand of what each circuit , 2£"-£*,™ 
ial Moat ^nsmrner. have one or more ^ ^ ^^ 

lould be' Lotn. Wn ntouto occur, the repairman should 
%£££U» the trouble front ab^o™ a, *a tn^on the 

£ ST y P 'Strtuiror Pr bu:d d tub,s ntay be found by 
ZZStJZ checafng f * co— ££££5% 

stages to the antenna. 

W l = r US antenna current wH, innate that 
-EF power is being delivered to the —a , ^^tna 
ren'rUrSL': ^cSS of the antenna on a 
d rjrii^„V:r s :. C Abno; m a, filantent voltage wil, Indicate 
the same troubles as for receivers (par. 114;. 

c. Plate voltage. Abnormal plate voltage will indicate the same 
troubles as for receivers (par. 114). ... . ,. . th 

d. Cathode voltage. Abnormal cathode voltage will mdicate 
same troubles as for receivers (par. 114). 



«,% L ,r ? latC ° Urrent - L ° W Plate current ma ^ indi ^te one of 
the following: 

(1) Insufficient excitation. 

(2) Soft tube (low emission). 

(3) Low plate voltage. 

(4) Grid voltage too high (negative). 

(5) Improperly loaded. 

«/' . H ! gh - Plate current - Hi ^ h P^te current may indicate one of 
the following: 

(1) Plate circuit not resonated. 

(2) Grid bias too low (negative). 

(3) Insufficient excitation. 

(4) Gassy tube. 

(5) Improper neutralization. 

(6) Wrong degree of coupling between stages. 

g. Rectified grid voltage. The rectified grid voltage of class 
C amplifiers will be normal if the grid circuit is properly tuned, 
if the plate circuit of the previous stage is properly tuned, if 
the coupling is correct, and, if the stage has sufficient excita- 
tion. Improper adjustment of any of these items will decrease 
the rectified grid voltage. 

136. TEST PROCEDURE. Figure 44 indicates a simple circuit 
for an AM transmitter with crystal oscillator and suppressor 
grid modulation of the power amplifier stage. All test points 
and part names used in paragraphs 137 to 141 refer to this 
circuit. Obvious modifications in test procedure are necos- 
sary to adapt this method of testing to other transmitters. 

137. POWER SUPPLY. This unit should be tested for normal 
output voltage while under load. However, a short circuit in 
a transmitter stage will cause an erroneous voltage measure- 
ment at the power supply. If the output under load is abnor- 
mal, continue to test the power supply as indicated in para- 
graph 116. 

138. OSCILLATOR. Check all tube element voltages. If these are 
normal, test the oscillator by measuring the voltage drop across 
Rl using a VTVM. If the crystal is not oscillating, replace it 
or readjust the oscillator tank circuit (LI and C2). A short 
in C3 or an open in CI would prevent the oscillator from func- 
tioning. Zero plate voltage could be caused by an open or short 
in the R2-R4 circuit, a short in CI, or an open in LI. 

76 



> 







-rw^rT 




«||, 







77 



139. BUFFER AMPLIFIER. If the buffer amplifier stage is func- 
tioning properly, a voltage drop, measured with a VTVM should 

aTtXT 83 ? ( u thG P r r amp,ifier is also operating and 
tl^ f 7^ V f^ geS Sh ° Uld be normal - If formal opera- 
tion is found, check C5, C6, R3, C4, C7, and L2. 

!tt°' f T ER + AMPLIFIER AND ANTENNA. Check the continu- 
ity of the antenna system, and determine if all tube element 
voltages are normal. If a trouble seems to be present, check 

grid bias battery ^ ° 12 ' M ' ™ *** fte su PP ressor 

141 AUDIO SECTION. Test methods outlined in chapter 6 for 
audio sections of receivers will be satisfactory for the audio sec- 
tion of the transmitter. The entire modulator including the 
microphone may be tested by connecting headphones across the 
secondary of T2 and listening for an audio signal while talking 
into the microphone. Measure tube element voltages, and check 
C15, R9, C16, R7, and Tl. Check the microphone circuits and 
the microphone battery. 



78 



CHAPTER 8 
REPLACEMENT REPAIR AND MAINTENANCE 

f rpnair and maintenance of radio 
142. GENERAL. Replacement, rep* r, ^a . ^ ^ d 

parts and equipment require confer ^^ re 

?he extent of such work ac^P^ed b^ ^ ^ ^ 

men will depend large y upor >^ Wch can be accom- 

Repairmen should start only *™**£e part is discovered, it 
plished with certainty When a defectiP ^^ and 
should be replaced wi h an tjenbcan^ ^^ to comply 
nected in an identical ma ^ ul ^ el guide d by the information 
with this rule, repairmen should It* ^f 
contained in the following paragraphs. 

• „„+ must be cleaned frequently 
143. CLEANING. Radio equipment J * ^ fa 

and thoroughly. Failure to do s0 \ contacts , worn mechan- 
roded parts and connections Jf«£* c , ean> dry air under 
isms, and, eventually, failure : ot cabinets, chassis, 

moderate or low 1™™™*"%*, be used for this purpose, 
and parts. A clean, dry cloth majn auo connectors must be 

Switch contacts, plugs, jacks, sock , ^ to maintal n 

periodically burnished wrth a £%%£» should be cleaned and 
a bright appearance. Moving mecn factur er's specifi- 

SMS* ss 3;£ ; — s. s 

<. 144. EXPOSURE TO SALT WATER. regions must 

be observed to protect ^ dampness. 

wate r due to subr»er B ,mr. spray, o da P ^ ^.^ ^ 

b Preventive me«isures. lo prevei 
form the g^^, in lld s and all plug.. Jacks, sockets, and 

connectors using scotch o r^ n d t h s eS ^th ta ?aseline or other neutral 
(2) Cover sliding joints win. 

grease. 79 



^S^j^^r.ar* after exposure of 

remoS and "pa*^ ST ^ "" ^ "^ "" ^ 

^h^l^^ or fIushi - with 

whichmay rusl "* PreVentIVe C ° mP0Und t0 meta,,ic P arts 

cloth and w? PartS are de ^' dry by Wiping with a clean 
c loth and heating in an oven. Field kitchen ovens may be used 

shn,n/h empera ! Ure mUSt n0t 6XCeed 135 ° F a ^ * Possible 5r 

avanable 6 IT"*™ ^T** ^ ° Ven ' If "° ° ther means ia 
ava,jable, the equipment may be placed beneath a cover and 

ttft^tTi^R" heat sources used t0 ralse the tem <- a - 

(5) Apply power for short intervals initially and watch 
tor flashovers or emission of smoke, which will indicate parts 
requiring: additional treatment. 

(6) As soon as time permits, after equipment has been 
exposed, thoroughly inspect and test as indicated in chapter 9. 

145. ELECTRICAL CONNECTIONS. Electrical connections must 
have mechanical strength as well as electrical conductivity This 
may be obtained by twisting wires together and tying wires to 
SI J "" 6 , ns bGf0re solderin S- Soldering irons must be 

K ,11 , G J" US6 . and ™ St be COrrectI y a PP lied ^ the part 
being soldered to avoid resin joints. Soldered connections must 
be neat and must not have large quantities of excess solder ad- 
hering to the connection. 

\i« u^ . WIRES - When repalrin * or re P la «"S wires, they 

Fail r/ t H 6 m °? l d fr ° m thGir ° riginaI P° sitions ^ the set 
Failure to observe this precaution may result in undesired feed- 
back or inductive and capacitive effects. Wires should be as 



. , i ;„ ..ltrahich frequency sets. High 
short as possible, particularly * n uI ™{* other lea ds unless ade- 
voltage leads should never be caDiea Vires should neV er 

quate insulation is used to prevent horting. ^ 

have sharp bends or other rf™*™ e to unusual strain, 
insulation or otherwise subject the wire 

147. RESISTORS. Defective r*to»™* ^^t high" 
sistors with the same ohmic resistar . g excessive 
wattage rating. When rectors bun l out. U« ^ immediate 
voltage or reduced resistance in t ne d and correcte d 
cause of the defective resistor mutbe^^ ^^ 

before replacing a part. If a P arucu n t res i s tor with a 
out and no unusual circums ances are present, 
higher wattage rating should be used.. 

148. COILS AND TIUJ^^JISJ^^^^ 
have the same inductance as the part rep • ^ 
must have the same turns "^ *VT £T opeJ receiver 
ondary impedance. An expedient «£« ^ The RF cho ke 
transformer primary is indu» ted i g ^ ^^ 

coil may be replaced by a resist0 ; °* , ' condary . The coupling 

^I^ri^^-i^cSTTo^n^^ to an RF stage 
condenser should be .0001 to .w» "' 
or .01 to .1 microfarad for an AF stage. 

OUTPUT 



INPUT 




FIGURE 45. 



Expedient repair of transformer primary. 



149 . COND.N SEHS ^J^SST^ l" »e £ 
AC voltages may be applied to them. An 



81 



coupling condenser may be replaced is indicated in figure 46 A 
few turns of insulated wire connected to the primary and wrapped 
around the grid lead of the secondary provide the desired cou- 
pling when a condenser is not available. 



INPUT 




FIGUKE 46. Improvised coupling condenser. 

150. TUBES. Vacuum tubes must be replaced by tubes of identi- 
cal types. When used in calibrated instruments or balanced cir- 
cuits, the replacement tube must be matched with the old tube 
or recalibration and reneutralization will be necessary. 

151. FUZES. Replacement fuzes must have an identical current- 
carrying capacity as the fuze replaced. When fuzes burn out, the 
cause of high current must be discovered and remedied before 
replacement. Circuits should never be overfuzed. 

152. RELAYS. Relays may require readjustment of air gap, ad- 
justment of armature tension, or cleaning of points. The air gap 
should be changed only when an adjustment is available and then 
should be set to the manufacturer's specification. Spring ten- 
sion on the armature may be adjusted when defective, but should 
not be changed unless definitely required. Relay points should 
be burnished when sticking or arcing; however, only a special 
relay point burnishing tool or a riffler file should be used. A 
flat file, sandpaper, or emery cloth must never be used. 

153. ANTENNAS. Special antennas and transmission lines re- 
quire special knowledge and equipment to construct and tune 
and should be avoided except when special applications require 
their use. Manufacturer's specifications regarding the antenna 
and lead-in must be rigidly followed. 

82 



154. GENERATORS AND DYNAMOTORS. ^ ^^ but onl 

a. Lubrication. Bearings must De^p^,^^^ Greage and 
in accordance with manufacturer s i commutator or brushes, 
oil must never be allowed to get on h &nd bright 

b. Commutator. The commutator should ^ ^ ^ ^^ 
If the commutator becomes rough or v ^^ or carborundum 
by using No. 000 or finer sandpap ^ ^ . 
cloth must never be used because , p tinue & lapping 
become embedded in the bars or b ™ ommutat or life. Insulat- 
action that greatly reduces ^brush and com ^ ^^ ^.^ 
ing mica must be undercut si igM y t i undercut the mica , use 
the mica and the brushes. If requirec i ^ width and 
a hacksaw blade which has been ground* t p commutat 
proceed very carefully £ ^ a t r remove all particles of 
bars. After dressing the commut 

abrasive by blowing 0U V V ] * f ' brU shes must be replaced im- 
c. Brushes. Worn or defective bw nes sanding 0f fi]ing 

mediately. A tight b J us Vr"oper elegance is obtained. When 
the sides of the brush unt 1 I P r °P er c ; e ted by placing a strip 
new brushes are installed they must oe gh ^ the com . 

of No. 000 or finer sandpaper **^ ^ brush and pulling 
mutator with the abrasive surface ^tac^g ^ ^ ^ .„ 

the sandpaper back and lortn. i - id of the brush so 

close contact with the commu ator J b oth » jf ^ brugheg 

that the corners of the brush will not be r ^ ^^ ^ 

are improperly seated, arcing will occu r interference with 

life of brushes, pit the ^^*V«rborundum cloth. Clean 
radio reception. Never use emery or 

thoroughly after seating the brusn , rf generat 

d. Generator adjustment. The °utp lue . manufacturer's 

3£M^3££ ^ this adiustment 

"fective, with an identical tjpe. *e P 

15 c. BEPnonucEns. ^£*g£2?%?£t T£ 

I57 . „ C.ncmTS . I. — ^ ««£«2££ 
^ntJ^nof b^ore than one,,* - the in— 



be us'ed 1 in'f mf 6 " "f^f * ^ tr ^^- Resistors should 
small ^ ° nly WhGn the Current flowin * j * very 

158 BRIDGING STAGES. Certain receiver stages mav be 

fo" fhe I, 6 ^ r^ ;V h i circuits «• chan g eftn 0m p a e y nsat 
lor the stages eliminated. RF and IF amplifier stages mav be 

B r rtot^fT eCtinS , the ? atG ° f the P-ed" fsSge to t h 
and conn^H T g B *«* I throu <* a -005 microfarad condenser, 
and connecting the cathodes together if not already grounded 

denser aT ? 7k -f^ ^^ USG a - 01 -^rofarad con- 
denser. A typical bridged stage is indicated in figure 47. _ 



-BRIDGING OF STAGE 



OUTPUT 




FIGURE 47. Rridging a stage. 

159. BRIDGING AM DETECTOR. The AM detector stage may be 
eliminated by bridging from the plate of the last IF amplifier to 
the grid of the first AF amplifier and providing sufficient nega- 
tive bias for the AF amplifier to cause it to operate as a plate 
detector. 



81 



CHAPTER 9 
SERVICE METHODS 

Paragraphs 
160-161 

Section I. General ZZZZZZZ." ••■• }$%-}%£ 

II. Receiver lbo-iso 

III. Transmitter 

SECTION I 
GENERAL 

m . GENERA, K~"^— e ™ = ^ 

service check may be required by the comm a g 

periodic intervals (preferably, ^^^^ £* op in 

hours of operation, whichever is least) , trouble may I 

the set. When frequency '^^^St^ioM 

permit a complet e -vice check the pairm ^ ^ ^ 

inspect each «t visually and ctertittP shou]d be 

as possible. Routine service checks o trouo 

as complete and thorough as Umj ™f ' -^^oubles. An 

to disclose impending troubles as wen d i i 

historical record should be maintained for ea ^ h ^ of a „ in . 

organization. This ™£^J^£X** the life 
spections, repairs, maintenance, etc., pt 

of the set. 

in <FRVICF ROUTINE. A definite service routine for both 
161. bLKVlLfc K"u' '" . „„; r0 fl This routine should in- 
receivers and transmitters is requ red. T^o ^ 

cl»de:a^»^^n^«.J^^ where necessary; 
sary; replacement repair, or i indicate a gen- 

and a final operating check, ^^^n^tteni respec- 
eral form to be followed for rccenen, a anplicable to each 

tively. This must be varied m deta ^ te »PPj«b 
individual type of radio equipment • Wh^^ ^^scover 
is encountered, the firman shwldaUajs ^ ^ 

8.> 



a report on the trouble found. If the radio set is used for vehi- 
cular operation, the vehicular installation must also be inspected. 

SECTION II 
RECEIVER 

162. PRELIMINARY INSPECTION. 

a. Purpose. The purpose of the preliminary inspection is to 
obtain information which will indicate the performance of the 
equipment, the physical condition of the equipment, and, possi- 
bly, symptoms of trouble which will simplify the process of 
trouble shooting. 

b. Procedure. 

(1) Obtain a report on performance. 
^ (2) Inspect for surface defects, such as, loose or misplaced 
wires, bent or broken parts, broken tube caps, defective wires 
or cables, odors, and cleanliness. 

(3) Check the lubrication of parts requiring lubrication. 

(4) Turn the receiver on and note whether the tubes glow 
or get warm, or whether unusual noises or odors develop. 

(5) Tune the receiver across the band. Check the opera- 
tion of manual controls and the operation of the receiver. Tune 
in signals at all points on the band. 

(6) Item (5) above should indicate a good receiver or one 
of following faults: dead, distorted, noisy, hum, oscillation, broad 
tuning, weak, fades, or operates intermittently. 

(7) Proceed as indicated in paragraphs 163 to 172 accord- 
ing to the type of operation noted. If symptoms of trouble are 
present, use the common trouble charts in paragraphs 173 to 183 
inclusive, to assist in isolating the trouble. 

163. GOOD RECEIVER. A good receiver receives signals on all 
frequencies without distortion. It has adequate sensitivity and 
selectivity, and all mechanical devices operate smoothly. Opera- 
tion is checked by tuning in signals as in paragraph 162 above. 
Adequate sensitivity and selectivity can be discovered by com- 
parison with a receiver known to be good, by comparison with 
previous experience with receivers of the same type, or by re- 
adjustment to maximum. If the receiver is apparently good pro- 
ceed to the final operating check (par. 184). 

86 



164. DEAD. .-. bv the circuit disturb- 

a. Isolate the defective stage or circu w 12? _ 
ance method (par. 132) or by the signal tracer m 

130), if possible. voltage and resis- 

b. Isolate the defective stage or c rcu^y ^ ^ ^ {n a &ho ^ 
tance measurements (par. iid-n°;» 91-102). 

c. Test the defective stage and .sola e cimnt par. 9 

d. Test the defective circuit and isolate par (par 69 

e. Replace or repair the defective part (par. 142 lo9, 

165. DISTORTED. # * . f or after the 
a. Determine whether the def f ^ f^f J27-130), and isolate 

detector by the signal tracer method (par. U< 

the stage, if possible. vnltaee and resistance 

h. W-.«-.*J«?5S?5f SS.5 ao^n a above, 
measurements (par. U6-i^) " 91-102). 

c. Test the defective stage and isc .late ■ ci«mt (F«r. 9 

d. Test the defective circuit and isolate part (par 6 90) 

e. Replace or repair the defective part (par. 142 159). 



166. NOISY. rtll fdHp the set by shorting 

a. Define if *"J*™£?£2£, * It shou.d cease. 

the antenna to ground K the no.se by ^ 

J^^t^? ^ tracer method <pa, 

d. Check for loose connections by wiggling wires P 

a screwdriver of insulating material. 142 _ 159 ). 

e. Repair or replace the defective part (par. 142 

16 I: "oil the defective stage by the stage muting method (pa, 
^ Check the defective stage for imperfect filtering in the 
power supply (par. 77 and 90). d , (par> 52) . 

c. Check the defective stage fo : mperte 

d . Test parts by substitution E ^ J^ ^ batterie3 . 

e. Check the power supply by repiacn k 

f. Correct the trouble. ^ 



168. OSCILLATION. 

^a. Isolate the defective stage by the stage muting method (par. 

J' Che * the d ^ctive stage for poor grounds, open grids, open 
bypass condensers, or open filter condensers. 

c Check the tubes by substitution. 

d. Correct the trouble. 

169. BROAD TUNING. 

a. Check the alignment of the RF and IF sections (par. 111). 

b. Check voltages in the RF and IF stages (par. 120-133). 
c Check the tubes by substitution. 

d. Check for high resistance in signal circuits (par. 69-74). 
^e Replace or repair the defective part, if required (par. 142- 

170. WEAK. 

a. Check tubes, by substitution or with tube checker. 
127ll3(!r k the AF StageS Using the signal tracer method (par. 

c. Check operating voltages (par. 113-126). 

d. Check for opens in signal circuits (par. 69-74). 

e. Check alignment (par. 111). 

f. Correct the trouble. 

171. FADING. 

a. Check for loose wires or corroded connections (par. 48). 

b. Check for leaking or shorting condensers (par. 44). 

c. If the volume drops but distant stations can still be heard, 
check the detector and AF stages (par. 98-100). 

d. If a distant station cannot be heard, check the RF and IF 
stages (par. 92-97). 

e. Check the tubes by substitution. 

f. Correct the trouble. 

172. INTERMITTENT OPERATION. 

a. Check for mechanical defects, such as loose connections, rub- 
bing tuning condensers, poor contacts, and resin joints. 

b. If jarring starts or stops the trouble, check connections care- 
fully. 

88 



, Locate whether the trouble ^before ^«er *e «ec£ 
by the signal tracer method (par. 121-lM) anu 

"Oscillator padder condenser W cause troub.e at the 

low end of the band. . , , f 

e. High converter bias may cause trouble at the, high end of 

of the band. «„**»» cnnnd in 

f. AF oscillation (motorboating) causes a '^t^™^ 
the reproducer. Check the AF section for oW ****** »nd 
sers, open grid circuits, or leads moved from their usual p 

g. If the defect occurs within three ^jnutes after the set 
turned on, check the tubes and power supply (par. 47 ^an I 83 JUJ 

h. If the defect occurs in three to five minutes, check resistors 

'Tifthe defect occurs after five minutes, check transformers 

( TTo 6 make thermal troubles appear' sooner, wrap the set in 

heavy paper. 

173. COMMON TROUBLES. Paragraphs 174 jo m ^^ 

list common troubles K^^Z^^^^^ 
may cause symptoms as indicated. T^^™ 3 metho ds of 
as a guide to be used in connection with standard methods 
trouble shooting. 



89 





T3 

Q 


•8 

o 
+-» 

.a 
Q 


.a 

o 


6 


PI 
.2 

o 

o 


bo 

g 
'3 

o 

In 

PQ 






4-> 

a 

CD 
+-' 

+-» 

*6 

CD 

a 


174. POWER SUPPLY. 
Battery defective 


X 


X 
X 
X 


X 


X 


X 
X 


X 
X 


X 
X 


X 
X 
X 

X 


X 


Input voltage high 


Input voltage low 










Input voltage fluctuates 












D ynamotor defective 


X 
X 


X 
X 










X 

X 

• X 

X 
X 


x 


Generator defective 










x 


Generator poorly adjusted 












Vibrator defective 


X 


X 
X 


X 
X 


X 
X 

X 
X 
X 


X 


— 


X 


Transformer defective 


Transformer secondary open 

Transformer secondary shorting... 
Transformer secondary half open.. 


X 
X 








Shielding defective *... 










Rectifier defective 


X 






y 


Rectifier weak 


X 








X 
X 


X 


y 


Buffer condenser defective 






X 

X 
X 






X 


Buffer condenser shorted 


X 




X 
X 








Buffer condenser leaking 




Filter condenser defective 




X 


y 


Filter condenser open 




X 




X 




Filter condenser shorted 










Filter choke defective 








X 
X 






y 


Filter choke open 


X 










X 


X 




Filter choke shorted 




Filter choke grounded 


X 
X 


X 
X 






Fuze defective 












Fuze holder loose 


y 


Voltage divider open 


X 


X 
X 


— 


X 








Voltage divider resistor defective.. 


y 


Wires or connections grounded 

Wires or connections loose 


X 








X 




Wires or connections intermittent-. 














y 


Wires arcing to ground 






X 














Power cord defective 


X 
X 




y 


Power cord connector defective 












X 


X 


X 



90 







£ 
a 

xn 

o 


e 
'S 

+-» 

•d 

a 
o 


•3 


8 






Q 


■*-> 

J-l 
o 
■*-> 

tffl 

S 




g 


G 

Qi 


- 










175. REPRODUCER. 


X 


Headphones defective 














X 
X 
X 






Headphone magnet weak... 
















Headphone diaphram bent 
















Headphone diaphram tight 










X 




X 


Voice coil defective 


X 










X 


X 




voice con open-~--«- — ---- 




Voice coil partially shorted 

Voice coil rubbing 




X 


X 








X 


Field coil defective 
















X 

X 
X 




Field coil intermittent 














X 




Cone improperly adjusted 




X 
X 












Cone loose at nm - - 






Cone torn __-----------"~ 


X 
X 




Cord open- __«---------•" 

Cord short 


X 


Cord intermittent 






X 

X 
X 










X 


Connections loose 




X 


X 






X 




Connections high resistance 








X 


Plug contact loose 


X 


X 








X 

X 
X 


X 

X 
X 


X 


Transformer defective 


X 


X 
X 






Transformer open 










Humbu eking coil shorted 




X 






Overloaded 






Feedback high 

176. TUBES. 


X 


X 








uurnea out _.---------*■ 

Gassy 




^Weak 

Cathode leakage 




X 
X 




X 








Microphonic ..-------- 






Element open -- 

Element shorted 






X 


X 


Element loose 

Oscillator not functioning 














X 


X 


Oscillator bias high 




X 






X 




X 




Shield loose or missing 








X 




















91 





T3 
OS 
CD 

o 

X 
X 


T3 
CD 

+-> 
»-l 

o 

.S3 
Q 


>> 

.23 

o 


s 


.2 
.52 
'o 

o 


g 
'3 

o 

t-l 

m 


J4 

CD 


8 


4-» 

PI 

CD 

1 

CD 
l-H 


Prong contact open 


X 
X 


X 
X 












X 


Socket connection open 








X 


X 
X 

X 


Socket connection loose 








X 


Socket connection high resistant 












X 


X 
X 
X 


177. FILAMENT CIRCUIT. 
Power supply defective 


X 












Voltage high ^ 


X 
X 






X 


X 




Voltage low 










Rheostat defective 




X 
X 

X 








x 


Connection defective.. 


X 


X 










X 
X 


x 


Connection high resistance 


X 






X 




• Connection loose 








x 


Wire shorted or grounded 


X 
X 




X 






X 






Wire open 




X 








Wire partially open 


X 


178. GRID CIRCUIT. 
Bias incorrect 


X 






X 




X 
X 


X 




Bias high 


X 
X 




X 




Bias low 






Bias resistor defective 






Resistor open 




X 










Resistor intermittent 




X 








x 


Resistor shorting 














x 


Coil open 




X 
X 










X 
X 


X 
X 




Coil grounded 














Coil intermittent . 




X 








x 


Bypass condenser open 




X 






X 


X 




Trimmer condenser defective 










x 


Tuning condenser shorting 






X 








x 


Tuning condenser rotor poor contact- 












X 

X 
X 
X 


X 

X 
X 


X 


Shielding defective 








X 


X 


X 




Leads too close to plate leads . . . 










Alignment incorrect 


X 










Circuit open 






X 


X 




Circuit grounded 


X 








Connection defective 




X 





X 


X 


X 



92 



• 


Q 


a> 
o 
$ 

Q 


o 


a 




s 
o 

X 


c 

*3 

•s 

o 


•a 

X 


8 

•a 

Pn 

X 

X 
X 

X 




<D 

1 

<D 

•3 


, . ™ ■ 




X - 
X . 




179. PLATE CIRCUIT. 
Voltage high 




Voltage low 

Resistor open 


X 


X 








X 


Resistor intermittent 

Resistor grounded 

Coil open 


X 
X 


X 


X 








X 


X 


Coil intermittent 

Coil shorted 

Choke coil open 


— 


— 


X 


X 


X 


X 
X 




Bypass condenser open 

Bypass condenser shorted - - -^ 

Tvitvimor prmdenser defective — - — 


X 






X 
X 


Tuning condenser shorting 






X 






X 


X 

X 
X 


X 
X 

X 




Alignment incorrect 






X 






Filtering defective 

Circuit open ' 

Circuit grounded 


X 






X 


Connections defective 

Connections ^ose.--- ^- ------ 

180. COUPLING CIRCUITS. 

Coupling condenser open 


X 
X 


X 


X 


X 


X 


X 


X 
X 
X 


X 

X 
X 


Coupling condenser shorted 




X 


X 








X 
X 


AF transformer secondary open-~ 








X 








X 


Insulation defective 










X 
X 

X 


X 


X 

. X 
. X 


. X 




Circuits tuned to wrong frequency . 












Circuits tuned too sharply- ------ 

181. DECOUPLING CIRCUIT. 








. X 


. X 


Decoupling condenser open 


X 






. X 


Decoupling condenser shorted 

■no/vMinlmcr condenser leaky 


. X 


X 








. X 
X 


Decoupling condenser intermittent. 


. X 


. X 














Decoupling resistor open 

Decoupling resistor value changed 


X 


Decoupling "*«**«■ intermittent.^ 


















0' 





Q 


"8 

o 
.S3 
Q 


o 


6 


■. 

.2 

a 
w 
O 


c 

3 

2 
m 




3 


c 

CD 

1 


182. ANTENNA CIRCUIT. 
Grounded 


X 
X 
X 


X 


X 


X 






X 
X 


X 




Coil open 






X 


Lead-in open 




Lead-in grounded 




X 


X 






Lead-in coupled to coil 










Lead-in intermittent 






X 




v 


Counterpoise open 












X 
X 
X 




Switch defective 


X 












X 


No ground connection 






X 


X 





Connections loose or dirty 




X 




x 


Antenna strikes trees, etc 
















x 


Antenna near source of noise 






X 






X 




X 
X 
X 

X 












183. MISCELLANEOUS. 


X 
X 








X 




Screen grid bypass shorted 

Volume control defective 


X 
X 


X 






AVC resistor open or changed 














AVC condenser defective 
















Regeneration 




X 


X 
X 


X 
X 




X 


X 
X 
X 




Shielding defective 






Intermittent contacts 








Wires misplaced 








Excessive signal strength 










Oscillator aligned improperly 














Switch contacts defective 


X 














Leads shorting to chassis 












X 



184. FINAL OPERATING CHECK. 

a. Purpose. The purpose of the final operating check is to de- 
monstrate that the receiver is performing satisfactorily and is 
ready to be returned to service. 

b. Procedure. 

(1) Turn the receiver on and tune across the band, noting 
the absence of unusual noises and squeals. 

94 



, a ,hPck the operation of the manual 

(2) Tune in a signal and check tne o V 

volume control. . AVC on strong signals where 

(3) Check the operation of the A v u 

the receiver has this feature. both ends of t he 

(4) Check the calibration of the aia 

band. ... .. hv tuning in a weak signal and 

(5) Check the sensitivity by tuning ^^ to be 

comparing the performance to that 

g0 ° d " (6 ) Checks select^ by ^^£rC 
to differentiate between signals on «U di the stren gth 

facturer's specifications »« rt * "^J ad j a cent channels, 
of signals which can be separated when on , ^^ fa 

(7) Check the s^^^gence through the receiver 
enough to assure passage of all intent 

• (see par. 163) . crating checks with the installation 

(8) Perform all a™ 1 .^™^ is to be used, 
and the antenna with which the receiver 

SECTION III 
TRANSMITTER 

185. PRELIMINARY WWE ^° N " Uinlnaiy inspection is to 

shooting. 

b. Procedure. ^^.p 

(1) Obtain a "port on Performance or m . gplaced 

(2) Inspect for surf fcce «^ ^ defective wire3 or 

wires, bent or broken parts, broKen 

cables, odors and cleanliness requiring lubrication. 

, (3) Check the lubncatioh oi parts r ^ tubeg 

(4) Turn the transmitter on ana no 

glow or get warm. the ban d r tuning the an- 

(5) Operate the transmiUer ^^ of manual contr ols, 
tenna when necessary. ChecK ine uy 

microphone, and meter readin * J. ith nor mal values. 

(6) Compare all meter readings wi h & t itter 

(7) Items (5) and (6) B ™*° .™ cn p r0C eed as indicated 
or symptoms ^J^SX^^ type of operation noted, 
in paragraph 18b 10 101 «* 9g 



naL T/ RANSMITTER - A g °° d transmitter transmits sig- 
nals on all frequencies without distortion. It has adequate power 
output and all mechanical devices operate smoothly. Operation 
is checked by tuning in signals on a receiver known to be good and 
noting operation. Relative power output is checked by measuring 
the effect of the transmitter on a good receiver and comparing 
with standards previously established, or by readjustment of the 
transmitter for maximum. If the transmitter appears to be good 
proceed to the final operating check (par. 188). 

187. DEFECTIVE TRANSMITTER. 

a. Meter readings, manner of operation, or visual inspection 
may indicate the nature of the trouble (par. 135) . If so, proceed 
directly to the defective stage or circuit and remedy the trouble. 
If not, proceed as indicated below. 

b. Check the power supply (par. 137). 

c. Check the operation and tuning of the oscillator stage (par. 
104, 135, 138). KV 

d. Check the operation, alignment, and neutralization of each 
amplifier stage, in turn (par. 103-107, 135, 137-140). 

e. Check the operation of the AF section (par. 141). 

f. Check the modulation system (par. 109-110, 141). 

g. Check the antenna (par. 60). 

h. Check individual parts as required (par. 42-68). 

188. FINAL OPERATING CHECK. 

a. Purpose. The purpose of the final operating check is to de- 
monstrate that the transmitter is performing satisfactorily and 
is ready to be returned to service. 

b. Procedure. 

(1) Turn the transmitter on and operate on all preset chan- 
nels or at the high and low end of the band when the frequency 
range is continuous. 

(2) Note that all meter readings are normal. 

(3) Set up a good receiver and separate it from the trans- 
mitter being checked by the rated range of the transmitter. 

(4) Connect a VTVM to measure limiter grid voltage if an 
FM receiver is used or AVC voltage if an AM receiver is used. 

(5) Tune the receiver and transmitter to the same fre- 
quency and operate the transmitter. Note clear reception with- 
out distortion of modulated signal, or clear reception of CW 
signals. 

96 



-a vnltaire or AVC voltage meas- 

(6) Record the limiter grid voltage or^ indlcation but 

ured on the receiver. Jh 1S of V ^7 transm itter power output. - 
not an exact measurement ot tne ^ gtand , 

(7) Compare the voltage ^.^^ a nsmitters of this 
ards established by experience with good 

type. , , . _ _ hecks with installation and 

(8) Perform all final operating checks * 
antenna with which the transmitter is to be used. 



97 



CHAPTER 10 
VEHICULAR INTERFERENCE 

Soiiom I. Causes and location of interference ^Ig^f 
II. Noise suppression "!"""" 19«99 

SECTION I 
CAUSES AND LOCATION OF INTERFERENCE 

189. GENERAL. 

a. High voltage surges or sparks set up by automotive elec- 
trical equipment may produce radio interference. Energy radi- 
ated from high tension leads may be picked up by the radio an- 
tenna, or high current surges may be conducted to the vehicular ' 
storage battery and from there to the radio receiver. Capacitive 
effects in the vehicle wiring system may cause the radiated energy 
to be very pronounced at certain frequencies. However, most 
interference is quite broad in frequency. 

b. Figure 48 shows a simple schematic diagram of a vehicle 
electrical system. 




FIELD 



AMMETER 
BATTER* 



98 



FIGURE 48. Simplified vehicle electrical system. 



190. IGNITION NOISES. The ignUion ^^]^^ 
of radio interference. Damped jr^^rks^reated at the dis- 
with the engine speed are radiated *^ «^™£ rk plugs . Large 
tributor rotor, distributor breaker pointo .and sp P ^^ 
current surges are also *»«£££ ttjvdu^ ^ ^ ^ 
by the ignition system. ™*™*™£ vet which vary in frequency 
ognized by clicking sounds in the receiver 

with the engine speed. 

191. GENERATOR NOISES. ^*°%X£^£* 
vehicle electrical system may ca us ^ n ^" e relay po ints. 
between commutator bars and pushes and between * ^ 
This type of interference causes a roarmg^ ^ ^ 
similar to alternating current hum. The am 

the brushes will increase under the following 

a. Brushes do not fit the commutator. . 

b. Worn brushes. 

c. Weak spring tension on the brushes. 

d. Oil or carbon on the commutator. 

e. Commutator worn irregularly (egg shaped). 

f. Excessive load on the generator. 

:„ fVio rwpiver are caused by 

192. BODY NOISES. Body noises in .^f. ^ n poorly grounded 

kets, and bolts are common causes of body noises. 

193. LOCATION OF NOISES. determine d by listening 

a. The type of interference should ^ ^termmed y 

to the noises in a receiver, ^isolate a part.cuiap ^ 

ment at fault, use the probe ^»2<S (stranded) and 

«A» is a 15-foot length of ^*^£?^ of No. 14 and 
"B'Ms a probe coil consisting of four to e^ ^ ^ 

No. 18 copper wire on a ""^^SeJk the conductor 
meter. One end of the probe ^ "J^Sthldded conductor 
and other end is open. The !**£*^*a short piece of 
and coil should be covered with ^^^^ han dling. 
wood fastened to the coil end of ^ cable I 

b. To isolate the trouble, connect ;the J™^"^ „„, ptace the 
tude modulated receiver as indicated in figure 



CONDUCTOR A 



:l 



ANTENNA* 
SCR - 284 
GROUND-* 




SHIELD J 



=d 




■^ 



D 




FIGURE 49. Probe type antenna. 

probe near the suspected wire, apparatus, or part. As the probe 
approaches the source of trouble, the noise will increase. To de- 
tect body noises, place the probe near the suspected part and then 
ground the part with bonding cable — the noise should decrease. 

SECTION II 
NOISE SUPPRESSION 

194. SHIELDING. Interference due to waves radiated from wir- 
ing may be prevented by shielding. Battery, generator, distri- 
butor, and ignition coil wiring is enclosed in a metallic sheath 
which is grounded so that interfering radiation can not reach the 
receiver antenna. Shielding should be used only with electrical 
equipment for which it was designed and which was designed to be 
used with shielding. 

195. SUPPRESSORS. Suppressors (inductive resistors) may be 
placed in distributor circuits as indicated in figure 50. One sup- 
pressor is in the distributor high tension lead and one in each 
spark plug lead. All suppressors must be carefully matched and 
should have approximately 10,000 ohms resistance. 

196. FILTERS. Inductive-capacitive filters may be placed in cir- 
cuits as indicated in figure 50. Nomenclature used indicates 
standard signal corps filters. 

197. BYPASS CONDENSERS. Bypass condensers may be used 
to eliminate noises when other methods are impossible, or may 

100 



• * not,-™* Recommended capacities 
be used to improve other installations. Kecom 

and locations appear below. 5 mfd 

a. Ammeter to instrument panel ^ ^ 

b. Regulator armature terminal to firewall 

c. Regulator battery terminal to firewall 

d. Generator armature terminal to genera- g ^ 

tor frame 

e. Horn line, at bottom of steering column, g ^ 

to column 1 m fd. 

f . Battery side of starter switch to firewall 

IGNITION COIL 




--VAA- 



SPARK 
PLUGS 



I W/-L 

L 10.000 TO 15.000 OHMS' ] 

MATCH tO i 



REGULATOR |||l' 



[ARMATURE] O 



FIELD 



,*_ FIGURE 50. Suppressed electrical system. 

108. BONDING. Bonding ^^£K"u£«^ 
of the vehicle are at ground potential A CMC 
points on %-ton or %-ton trucks is as follows. 

a. Front fenders to rear of splash apron. 

b. Headlights to frame. 

c. Radiator to cradle and cradle ,to .frame 

d. Hood, across center hinge and to firewall. 

e. Steering column to firewall. 



f. Left and right splash aprons to frame. 

g. Both sides of firewall to frame. 

h. Throttle rod, thermostat cable, speedometer cable and choke 
rod to firewall. 

i. Oil gauge line to firewall. 

j. Rear of cylinder head to firewall. 

k. Right and left rear of motor support bracket to frame. 

I. Left front of motor block to frame. 

m. Both sides of cab to frame. 

n. Both sides in rear of body to chassis. 

o. Rear of fenders to body. 

199. SERVICE METHOD. To isolate and eliminate vehicular in- 
terference, proceed as follows : 

a. Perform following operations: 

(1) Check the bonding, making sure that connections are 
clean and tight. 

(2) Clean the spark plugs and adjust their gap according 
to manufacturer's specifications. 

(3) Clean the breaker points and adjust the gap according 
to manufacturer's specifications. 

(4) Clean and check the generator commutator and brush 
assembly. 

(5) Clean plug-in and slip-on connections at the distributor, 
coil, and spark plugs. 

b. If steps in a above did not clear the trouble, isolate the nature 
of the trouble (par. 193). 

c. Add ignition suppressors, bypass condensers, filters or bond- 
ing according to the nature of the trouble until eliminated. 

d. All work done on motor vehicles should be done by the motor 
mechanic and not by the radio repairman. 

e. Since some vehicle manuals contain information on suppres- 
sion, these manuals should be consulted before attempting to test 
an installation or repair a defect. 



102 



CHAPTER U 
RADIO SETS SCR-609 AND SCR-610 ragMpM 

201-203 

Section I. General »■■•••/•■■"■" "V* ;:".; •••■ ///.I. 204-20S 

II. Presetting ^Neutralization 209-212 

III. Alignment and neuuau 

IV. Servicing 

SECTION I 
GENERAL 

200. GENERAL. _ frequency modulated 

a. Radio Sets SCR-609 an ^ SCR-610 a rej ^ . n ^ R dl0 
transmitting and ^^^Sonente required for vehic- 
Set SCR-610 includes additional comp 

ular operation. . arts method of in- 

b. Details concerning the /un^« f^ > re conta ined in 
stallation and operation, and .^"dSt" supplement TM 11-615 

me e!' K e to to *«. « «* «^*«- ^ £ S 

as^sir jt^ <" -«* *- « * hfch ocoom ' 

panies any particular set. 

SECTION II 
PRESETTING CHANNELS 

201. PREPARATION. removing ten screws 

a. Remove the '^» ^£ t ^ c l2U forward, 
from the front panel and pulling : tn ^ Qf ^ 

v b. Place the chassis on a metal shell 
power supply case. 

c. Set switches SW-1 and SW-2 O . ^ name 

d. Insert the proper crysta 1=> > n ^/ge socke t. 

plates facing away ^^^ by inserting the DC volts 

e. Check the interna battery BA 4 y g socket ( th 
probe of a VTVM in pm ifj^ les8 than +20 volts ,ndi- 
common lead grounded ). Am * been instalIed . 

cates that the BA-41 is weak or n ^ 



f. Connect the power and control cable plug on the chassis to 
the receptacle of a good power supply. The cable must not be 
twisted during this operation. If dry cell batteries are used, place 
Adapter RS-259 in series with BA-39. 

g. Set the panel meter switch to CHECK. 

h. Tighten lock nuts on the tuning condensers until a snug fit 
is obtained, but not tight enough to make adjustments difficult. 

i. Set all tuning condenser indexes to appropriate setting shown 
in the chart below. If the index is difficult to read, set the con- 
denser to maximum capacity and the end of the slot pointing to- 
ward zero will be the index end. 





APPROXIMATE 


TUNING 


CONDENSER SETTINGS 




Channel 


Al-Bl 

Rec. 
Osc. 


A2-B2 
Mixer 


A3-B3 
R-F 
Grid 


A4-B4 

Trans. 

Osc. 


A5-B5 
Buffer 


A6-B6 
P-A 
Grid 


A7-B7 
P-A 
Plate 


270 


0.0 


1.0 


0.8 


0.2 


0.4 


0.0 


1.0 


280 


0.6 


1.9 


1.4 


1.1 


1.2 


0.7 


1.8 


290 


1.4 


2.4 


( 2.0 


1.7 


1.7 


1.3 


2.4 


300 


2.2 


3.0 


2.4 


2.3 


2.2 


1.8 


2.8 


310 


2.9 


3.6 


3.1 


2.8 


2.7 


2.3 


3.1 


320 


3.6 


4.1 


3.8 


3.5 


3.3 


3.0 


3.8 


330 


4.0 


4.2 


4.2 


3.9 


3.7 


3.4 


3.9 


340 


4.1 


4.6 


4.5 


4.2 


3.9 


3.7 


4.0 


350 


4.6 


5.0 


4.7 


4.5 


4.0 


3.9 


4.1 


360 


4.9 


5.2 


4.8 


4.8 


4.5 


4.2 


4.2 


370 


5.4 


5.5 


5.2 


5.2 


4.9 


4.6 


4.7 


380 


5.6 


5.6 


5.3 


5.5 


5.0 


4.8 


4.8 


389 


5.8 


5.8 


5.4 


5.9 


5.1 


5.0 


5.0 



j. In adjusting tuning condensers in paragraphs 186 and 187, 
if it is necessary to move far from the approximate setting, check 
previous operations and examine the condensers for misalignment 
of plates or other defects. 

k. Plug in the microphone and turn the volume control to its 
maximum clockwise position. 

1. Preset both channels at the same time, setting the CHAN 
switch to A or B as appropriate for the condenser being adjusted. 

m. Proceed with the precise presetting operation if a VTVM 
is available or with the field method if a VTVM is not available. 

104 



202. PRECISE METHOD. . rat j n g instructions 

a. Set up a VTVM in -e^^^ lead to the 
for measuring DC voltages and connect tn d from the 
chassis of the set. Note that the panel 

chassis of the set. . . . No i f the metering 

b. Insert the VTVM probe ^^J d indicat e -30 volts 
socket (300 volt range). The VTVM stiou . g defective . 
or more. If not, the crystal j>r crystal o^Uator ^ ^.^ 

c. Insert the VTVM probe , n p in J ^ * • maximum indica . 
socket (30 volt range). Adjust Al and m 

tion on the VTVM (-4 volts or more;. ^ range)# 

d. Insert the VTVM probe in pin ' 3f « No. V 

Adjust A2 and B2 for maximum .^f on on the ^ ^ ^ 

e. With the VTVM probe in pin jack No. 8, 

for maximum indication on the . Vi .^ g djust A7 an d B7 

f. With the VTVM probe in pin jack No. 8, adj 

for maximum indication on the Vivn. reading is ob- 

g. Repeat d, e, and f. Make sure that maximum reading 
tained after the aligning tool is ranoved. ^ 

h. Remove the first ^'f^J^lSL the micro- 
VTVM probe in pin jack No. 3 ! (30 voU ^r S ^ indlca tIon on 
phone switch and adjust A4 and JB4 ^f tot J t peak 

fhe VTVM. If more than one peak is found u^e microphone 

near the approximate setting f^J^^^g sound as the 
during this adjustment and lis n ^ or ^^^ is no t at the 
top of the peak is approached. If the ae ^ reallgned . 

top of the peak, the discriminator probabi ^n ^ ^^ 

i. Insert the VTVM probe in pin ack N • I __ 5 to 

Press the microphone switch and adjust A4 
-6.0 volts on the VTVM. volt range) . 

j. Insert the VTVM probe ir , pin . ax* Nol l maximum 

EsatStW^SfK^ 4 and prcss the 



volts, readjust h, i, j above ; y5) turn S W-1 ON, press 

1. Replace the RF amplifier tube ' l* '• b6 ^ niaxlmum 

the microphone switch, and adjust ter switch must be 

panel meter reading (1.5 or more;, r 

at CHECK. , de quickly to avoid 

m . The Mowing ^ st ™ p an e ltr"wi«ch to OPEH, turn 

damage to the tube. Turn tne pa ^ 



SW-2 ON, press the microphone switch, and adjust A7 nnd TC7 
for minimum panel meter reading (1.0 or less) 
n. Repeat 1 and m above. Make suro +w w,„- , . . 

to *fS:*,t^ s^s v ;r^ ■" thoy ar ° not * se 

- o i ,. , settings, the set is improper y tuned. 

P. Replace the chassis in its case and readjust A7 and R7 for 

minimum panel meter reading. eaajust A7 and B7 for 

<|. Extend the antenna and assemble it t the sot Press the 
ZTSSTtoiEZ. s n ° tC ' he "r ' meto ~*£ in" than 
the adSmem^AraX 8 ""'^ " m ° re *"*» "• "*«* 

? no fas »od M a E ,T » The *">«*« ! °< «««'»« *™* 

v™"LS,: b ,*ys a m ,tu„r should "* "» used " * 

a. Prepare the set as indicated in paragraph 201 

b If an approximate setting chart is not available adjust the 

srts estimation <— i <« ™> -; a 3 d for^: 

producer! 181 ^ ^ Bl f ° r maXimUm rUshing sound J « the re- 

d. Adjust A2 and B2, A3 and E3, and A7 and B7 in turn for 
maximum rushing sound in the reproducer. 

e. Repeat c and d above. 

f. Remove the first RF amplifier tube (V5), press the micr< 
Phone switch, and carefully adjust A4 and B4 for loudesTand 
clearest sidetone while blowing gently into the microphone 

g. Replace the RF amplifier tube (V5), turn SW-1 ON Dress 

maxrum P „a n n:r Vit t Ch ' ^^ M ^ B5 ' and A6 »"Sb6 to 
cSECrpoSn." 1 ^ readmg - Panel metGr SWitch ™ st be <" 

h. Repeat g above. 

i. Turn the panel meter switch to OPER, turn SW-2 ON, press 
the microphone switch, and adjust A7 and B7 for minimum panel 

w^r operation must be *- *«** ^ 

J™^!? h and i u b0Ve ' Make SUre that ma ximum and mini- 
mum readings are obtained after the aligning tool is removed. 

to tL R r heCk . aI1 Cond f nser settin ^ s v is^lly. If they are not close 
to the correct approximate settings, the set is improperly tuned. 




FIGURE 51. BC-659-A Transmitter and Receiver- Schematic Diagram. 



v i. «M in its case and readjust A7 and B7 for 

1. Replace the chassis in its ca* 
minimum panel meter reading. t() the case . Do no t 

m. Extend the antenna and «*» feads more than 2 . 5 . 

readjust A7 and B7 unless the panel m 

SECTION III 
ALIGNMENT AND NEUTRALIZATION 

204. GENERAL. discriminator stage should 

a. Alignment of the IF ^^"^et. When improper 
not be required each }^.^%^t ia n or by the preset- 
adjustment of either » ^^J ld fc taken at once. 

ting procedure, remedial action .no generator 

b. The signal source for ^ n ™f ^^ ^vhich will fit the 
which will produce 4.3 m c or a 43 m « ^_ B , g used> it should 
crystal socket of the set. If osc ^ t termina i. If Frequency 
be coupled directly to the ap *wpr »te by twisting one end 
Meter SCR-211 is used, it must o v ^ ^ f requency meter 
of insulated wire around the amen i insulat ion) to the ap- 
and connecting the other end (stripped 

propriate set terminal. amplifiers should 

c. Neutralization £ «^»™£ Stings of the neutralizing 
not be necessary unless the origin* 

condensers are changed. 

205. ALIGNMENT OF IP ^^ ndconnect toagoodWwer 

a. Remove the chassis from its case ana 

supply. , . the vo i U me control to its 

b. Plug in the microphone and turn the 
maximum clockwise position. 

c. Turn SW-1 and SW-2 ! OFF. ^^ &] 
^ d. Couple the output of the «gnal g ^ ^ of 
"No. 3 on transformer T2 an l & socVet (30 volt range), 

VTVM in pin jack No. 3 of the mete^ g ^ ^^ ^ adjugt 
ground the common lead oi ™* . VTV M indicates between 

the output of signal generator until the 



-10 and -20 volts. transformer T4 primary 

e. Adjust the adjustment screws ary and secondary , in 

and secondary, and transtorme J Pr i ma ries are on top 
that order for ^™ ™« ? n t he bottom underneath the 
of the cans and secondaries are 



chassis. Reduce the output of the signal generator as each stage 
is aligned to assure sharp peaks. 

f. Remove both crystals and couple the signal generator to the 
terminal leading from SW-10 to the control grid of the mixer 
tube (prong 6 of V7). 

g- Adjust the screws on transformer T2 primary and secondary 
for maximum VTVM reading. 

h. Disconnect the VTVM and signal generator, replace the 
crystals, turn SW-1 and SW-2 ON, and reassemble the chassis in 
its case. 

^ i. If the output of the signal generator can not be reduced suf- 
ficiently in above procedure, remove the crystals from the set and 
connect the signal generator to the lower contact of SW-10 and 
ground. When the CHAN switch is in B position, the generator 
will be coupled directly to the mixer grid. As stages are aligned, 
switch to A position to prevent overloading and consequent broad 
tuning. 

206. ALIGNMENT OF DISCRIMINATOR. 

a. Proceed as in paragraph 205 a, b, c, and i above. 

b. Insert the DC probe of the VTVM pin jack No. 8 (10 volt 
range). Adjust the primary trimmer of the discriminator trans- 
former (T5) for maximum negative reading. 

c Adjust the VTVM for false zero on the 3 volt range (par. 

d. Insert the DC probe of the VTVM in pin jack No. 7 and ad- 
just the secondary trimmer of the discriminator transformer (T5) 
for zero voltage within ±.25 volts (meter will read false zero). 

e. To be sure that the secondary is not adjusted to one side'of 
the correct frequency, tune the secondary slightly above and be- 
low the supposed correct adjustment and note that the meter 
swings both directions from false zero. 

207. ALIGNMENT OF IF AMPLIFIERS AND DISCRIMINA- 
TOR USING 4.3 MC CRYSTAL. 

a. Remove the chassis from its case and connect to a good 
power supply. 

b. Plug in the microphone, turn the volume control to its max- 
imum clockwise position, and allow the set to reach normal operat- 
ing temperature. 

c. Turn SW-1 and SW-2 OFF, remove both crystals from their 
sockets, and remove the first RF amplifier tube (V5) from its 
socket. 

108 



* , ■ tn nne crystal socket and turn the 
d Plug a 4.3 mc crystal into one W« 
CH'AN switch to its corresponding Pos^o^. .^ ^ 

jsj^ W££ - — he — Iead o£ 

the VTVM to the chassis. T4 primary and second- 

f. Adjust the screws on transio and sccondary , 
ary, T3 primary and secondary and 1 P 

in that order for maximum VI vu No g and ^. 

g. Insert the DC probe * t£^ M for ^ VTVM read- 
just transformer T5 primary trimmer 

" h. Adjust the VTVM for false jzerc > (par- 21c) ^ ? ^ 

L insert the DC probe ^^^J ? or 'zero voltage with 
just transformer T5 seconaar * 

assemble the chassis in its case. 

208. NEUTRALIZATION. g ^ to a good power 

a. Remove the chassis from its case an 

supply. . .„ , he clIAN switch to the chan- 

b. Plug in the microphone, turn tne u and turn the 
nel with the highest frequency, turn SW 

panel meter switch to CHbOK. B7 (depend ing 

c. Press the microphone > switch and Jtarn range> 
on the position of the CHAN swit h ^ougn ^.^ ^ 

d. If the panel meter -reading jips ^rnore t ^^ ^.^ 
ing c above, neutralization is necess ^ r ^ A7 or B7 for minimum 

e. Press the microphone switch and tune a< 

meter reading. , t e the neutralizing con- 

f. Press the microphone sw.tch nd . tune^ ^ chagsig) 

' densers C4 and C5 (found on either s jde ,ot and c5 shouM be 

for maximum meter reading. The settings 
-TRtete and fTn^the detained with A7 or B7 is less 
than one division. 

SECTION* IV 
SERVICING 

* Qrn_609 and SCR-610 should be 
209. GENERAL. Radio sets bO« ^ hQurg of operation , r 
inspected once each month, or a whiche ver is earliest, 

whenever defective performance occu ^ 



This inspection must include all auxiliary equipment, spare parts, 
and vehicular installations. In addition to this inspection, every 
opportunity should be taken to check the condition of the set 
when presetting channels. An historical record of all inspections 
and all maintenance work should be kept. 

210. PRELIMINARY INSPECTION. 

a. Obtain a report on performance. 

b. Vehicular inspection. Inspect the vehicular installation and 
perform the following operations : 

(1) Inspect the vehicle storage battery and note loose or cor- 
roded connections at the battery terminals. Check the state of 
charge of the battery. 

^ (2) Inspect the power unit supply cable. Connections must 
be tight, proper polarity of the cable must be observed in making 
the connections, the cable insulation must not be damaged, and 
the cable must be adequately anchored to the body of the vehicle 
to prevent strain due to unnecessary vibration. 

(3) Inspect Mounting FT-250. It should be firmly fastened 
to the body of the vehicle and the shock absorber device must be 
free from defects. 

(4) Inspect Plate Supply Unit PE-117-C. It should show 
no evidence of physical abuse, link connections should be 
properly made according to the voltage of the vehicle electrical 
system, and all spart parts should be good and mounted in their 
proper place. 

(5) Clean the power supply unit. 

(6) Inspect the antenna system. Three section mast an- 
tenna with a three foot lead-in of wire W-126 may be used. Con- 
nections must be firm and not corroded or rusted. The mast base 
must be firmly mounted on the vehicle so that the antenna may 
assume a verticle position, and the mast base insulator must not 
be painted. The antenna must not be tied down with wire, either 
insulated or uninsulated. If a coaxial cable feed system is not 
used, the coxial lead should be removed from the mast base. 

(7) Measure the voltage at link connector No. 18 of the 
power unit. For proper radio operation, this should not exceed 
7.1 volts with the radio set operating and the vehicle engine run- 
ning at a speed equivalent to 30 miles per hour on level highway. 

c. Receiver and transmitter. Inspect the receiver and transmit- 
ter unit and perform the f ollowing operations : 

(1) Inspect the set case for possible damage to fasteners 
and antenna terminal binding post. 

110 



* t^ m \u case and inspect switches, 

(2) Remove the set ^V*^ lou dspeaker. 
meter, jacks, rubber gasket desiccator a ^ ^ 

(3) Inspect the radio chas ^ *°/ *" iable condensers bind- 
loose or misplaced wires; imaged coisvambe C fw ^^ 

ing, shorted or with loose ^l^^lZrect tube type or 
contacts; defective tube socket contact , ^ ^^ 

tubes improperly inserted in the socKe , 
shields; bent or broken parts; or unuBual^ 

(4) Clean the receiver and transmitter unit 

d. Other eauipment. l^ h >£^^£Z&£* 
trol unit, collapsible antenna battery case, and a j 

or spare parts for evidence of physica dama ge or P^ ^ ^ 
condition. Clean when necessary and replace a 

'"iCation. With the receiver and «»—£'£%£ 

connect it to the 7**' ^j |™& Taxman, and 

engine so that the input to the power supp y 

check the following: _ , th rece iver ON, the 

(1) With the meter switch at FIL ana me 

panel meter should read 2 or more microphone 

(2) With the meter ^*™£l 2 or more. 

switch presed, the panel meter should reaa microphone 

(3) With the meter switch at CHECK ana^ i 

switch pressed, the panel meter -should read l*"™^^ 

(4) With the meter switch at or u.«- «" 
switch pressed, the panel meter should read M L or less 

(5) With the microphone JjJ^^^,^ w hen 
sidetone should be heard in the \ ouds P eaKer ° 

the repairman blows into the m > c ^° ne ' h k the fo i, ow ng volt- 

(6) With the set in receive position, checK 

tages at the meterng socket with a VTVM ^ ^ ^ ^ 

Pin jack No. 1 _ 4 vo]tg or mor e 

Pin jack No. 2 Dressed, check the follow- 

.,. (7) With the microphone si vitch pres sea ^ 

ing voltages at the metering socket ^jj™' a more 
Pin jack No. 3 _5 to -6.0 volt3 

Pin jack No. 4 _15 to -30 volts 

Pin jack No. 5 ' _8 to -15 volts 

(« tllalM receiver and transmitter in its case, con- 
nect the vehicular antenna, and /* n ™ microphone 

(9) With the meter switch , a OPER and^ g ^ 
switch pressed, the panel meter snouia rea 



(10) A loud hissing sound should be heard in the loud- 
speaker or the headphones when no signal is being received. 

(11) An incoming signal should be received without dis- 
tortion. 

(12) With the microphone switch pressed, an undistorted 
signal should be transmitted. 

f. Alignment. Defective presetting or alignment may not al- 
ways be discovered in these tests, but will be in the final operating 
check. If the presetting or alignment is known or suspected to be 
defective proceed at once to a complete alignment of the set (par. 
201-207). 

g. Neutralization. Check the neutralization of the power ampli- 
fier and reneutralize if necessary (par. 208). 

h. The preceding steps should indicate whether the set is good 
or defective. If apparently good, proceed directly to the final 
operating check. If symptoms of trouble have been discovered, 
locate and remedy the trouble before performing the final operat- 
ing check. Do not attempt repairs where successful completion 
is doubtful. 

211. COMMON TROUBLES. 

a. General. Common troubles of the SCR-610 are indicated be- 
low according to the symptom or indication they produce. This 
is not a complete detailed list and is to be used only in conjunction 
with standard service methods (chap. 9). 

b. FIL meter does not register. 

(1) Dead BA-40 or storage battery. 

(2) Defective connector plug contacts. 

(3) Open or shorted lead-in cable. 

(4) Defective meter or meter switch. 

(5) Defective PE-117-C. * 

c. FIL meter registers low. 

(1) Weak BA-40 or storage battery. 

(2) Defective connector plug contacts. 

(3) High resistance contacts at meter switch. 

d. FIL meter registers full scale. 

(1) Shorted wires in cable. 

(2) Defective meter. 

(3) Four or more burned out tubes. 

e. FIL meter registers in reverse. Battery connections re- 
versed. 

112 



f. PLATE meter does not register. 

(1) Defective fuze. 

(2) Dead BA-39 or storage battery. 

(3) Defective PE-H7-C. 

(4) Defective connector plug contacts. 

(5) Shorted wires in cable. 

(6) Switch SW-1 open. 

g. PLATE meter registers low. 

(1) Weak BA-39 or storage battery. 

(2) Defective PE-117-C. pnnnecto r plugs or meter 

(3) High resistance contact at connector Pi, 

switch. . , 

h. CHECK meter does not register. 

(1) Tube VI filament burned out. 

(2) Tube VI or V2 defective. 

(3) Switch SW-1 open. 

i. CHECK meter registers low. 

(1) Transmitter improperly aligned- 

(2) One half of tube V2 filament burned out. 

(3) Tube V3 defective. 

j. CHECK meter registers high. 

(1) Defective resistor Rl. 

(2) Defective meter. 

k. OPER meter does not register. 

(1) Tube VI defective. 

(2) Switch SW-1 or SW-2 open. 
1. OPER meter registers low. 

(1) Tube VI defective. 

(2) Defective antenna system. 

3 Antenna contactor not making contact. 
m . OP E t mlr registers high. Transmitter aligned miprop- 

er n. Voltage at pin jack No. 1 low (receiver ON). 
'*" (1) Crystal defective. 
(2) Tube V8 defective. 
o. Voltage at pin jack No. 2 low (rece.ver ON). 

(1) Crystal defective. 

(2) Tube V8 defective. 

3 Receiver RF oscillator stage «*£• 
p. Voltage at P in jack No. 3 low (transm.tter ON). 

(1) Receiver improperly aligned. 

(2) Transmitter improperly aligned. 

113 



q. Voltage at pin jack No. 4 less than 20 volts (receiver OFF), 

(1) Battery BA-41 defective. 

(2) Battery BA-41 not installed. 

r. Voltage at pin jack No. 4 not 5.0 to 6.0 volts (transmitter 
ON). 

(1) Transmitter improperly aligned. 

(2) Battery BA-41 defective. 

(3) Tube V4 defective. 

(4) Reactance modulator stage or DC amplifier stage de- 
fective. 

s. Voltage at pin jack No. 5 low (transmitter ON). 

(1) Tube V3 defective. 

(2) Transmitter improperly aligned. 

(3) Transmitter oscillator stage defective. 

t. Voltage at pin jack No. 6 low (transmitter ON). 

(1) Tube V2 defective. 

(2) Transmitter improperly aligned. 

(3) Transmitter buffer amplifier stage defective. 
u. Unable to hear other stations. 

(1) Defective reproducer. 

(2) Defective power supply. 

(3) Receiver improperly aligned. 

(4) Defective receiver tube or tubes. 
v. Unable to modulate transmitter. 

(1) Defective microphone or cord. 

(2) Defective tube V4 or reactance modulator stage. 
w. Unable to align transmitter. 

(1) Battery BA-41 defective. 

(2) Tube V4 or V13 defective. 

(3) Power amplifier improperly neutralized. 

(4) Defective discriminator adjustment. 

212. FINAL OPERATING CHECK. 

a. General. The final operating check must be a rigid test of 
the set's performance under conditions which will indicate its 
ability to receive and transmit satisfactorily at extreme ranges. 
Two methods are outlined in b and c below for this check. Either 
will be satisfactory if performed by experienced personnel who 
are thoroughly acquainted with the set's specifications and char- 
acteristics. 

b. Readability test. Install the radio set to be tested in the 
manner in which it is to be used. Set up a test set, which is known 
to be good, on the same channels as the set to be tested. Move the 

114 



« set .o a point seven n*s "^*f ^SttS 
average intervening terrain. ™ e h reception on each re- 
on each of its preset channels and note ™ e r P fo be easily 

ceiver. It should be «" dlstor ^^fi that it would reduce 
understood. « the intervening terrain is sucn maximum 

signal strength, good results ; can ^not _b exnec e fof 
range. Reduce the ^stance between stations to ^ ^ 

this variation from standard condition and p manner 

c. Voltage test. Install the ra dio ^£te toted ^ fa . 

in which it is to be used. Set up « SCR608tt ^ 

known to be good, on the same channds as t ne a _ 

(The SCR-608 test set ^ust be mod f*d as ^ } ^ 

graph 234 so that limiter grid voltage may ^ 
the test set to a point seven miles from the se 

*^, , 1SrS B ^o'Si^£ ^ ^uency and 

the SCR-610 is transmitting. with read . 

(3) Compare the ^ding obtained in ( 2^ ^^ 

ings obtained under similar Mn * tl0 ™ ^irfgon will give a 

""ES^E SSLT^S 4 .^^- 8iS na, ana 
check for lack of distortion at the test set. in 

(5) Move the test set to a po nt whe re tne b 
the SCR-610 receiver being tested is just strong enougn 
heard while the test set is transmi ^ } and check 

(6) ^^^^^^^i^i be readable 
for readability at the SCR-610 recener 

despite the noise level. This tests reaver sensitivity 

(7) Repeat (1) to (6) J^^^tSaln may effect 
^ (8) Since the nature of the in erven^to ^.^ 

results to a large degree, «c*n £ ™ e be 

range can not always be expected. Unfavorable 
considered when interpreting results. 



115 



CHAPTER 12 
RADIO SETS SCR-608 AND SCR-628 

e T „ Paragraphs 

Section I. General 213 

II. Presetting channels 214-217 

III. Receiver alignment 218-222 

IV. Transmitter alignment 223-229 

■V. Servicing 230-234 

SECTION I 
GENERAL ' 

213. GENERAL. 

a. Radio sets SCR-608 and SCR-628 are frequency modulated 
transmitting and receiving sets which differ only in that Radio 
Set SCR-608 includes two receivers and Radio Set SCR-628 in- 
cludes only one receiver. 

b. Details concerning the functioning of parts, method of in- 
stallation and operation, and maintenance data are contained in 
TM 11-620. These notes are intended to supplement TM 11-620 
and to provide field artillery repairmen with certain items of in- 
formation of value to him in maintaining equipment. 

c. Refer to figures 52 and 53 for identification of all parts and 
test points. Since more than one model of this set is in use, these 
schematic diagrams may differ in details from those which ac- 
company any particular set. 

SECTION II 
PRESETTING1CHANNELS 

214. PREPARATION OF RECEIVER. 

a. Remove the left hand receiver from its mounting. 

b. Release all push buttons by partially depressing any one of 
them. 

c. Turn the power switch ON and allow the receiver to stabi- 
lize for 15 minutes. 

d. Turn the SQUELCH switch ON, SENSITIVITY control to 
minimum (counterclockwise), SPEAKER switch ON, and TUNE- 
OPERATE switch to TUNE. 

116 



TT , , ^ *, ptt«?W TO-TUNE button and rotate the TUN- 

e. Hold down the PUSH-1U i u» j the word 

ING control manually to the high end of the 

LOCK appears on the CHANNEL d' al - b h but tons, 

f. Turn the selector locking ^/^^^ then clockwise 
as far as possible in a counterclockwise direct, 

one half turn. ^tothittfi? 

215. PREPARATION OF TRANSMITTER 

a. Release all push buttons. crystal com- 

b. Select desired crystals and insert them in 7 

partment. 

c. Plug in the microphone. TUNE-ANT CUR 

d. Turn the transmitter power sw.tc h ON TUH yER . 
switch to TUNE, METER switch to .No. 1, 

TUNE OPERATE switch to 0PbK ^; , clockwise to reduce 

e. Adjust the antenna coupling control cIockw 

coupling about one half. control, located on the right 

f. Turn the gang condenser tuning contro, ^^ 
end of the transmitter near its ba seta th f ran p ^ 

the selector locking screw, and return the contro 
tion. 

*t Sri— ter av„a m otor for short period, 0* to 
P TfZ"Xr£s a transmitter push battoa Wore pressing 

the microphone switch. adjustments must be made 

c. Antenna tuning and COU P h "f ^ J " on ^ected to the antenna 
with the set mounted in the vehicle and connected 
which is to be used in later operation. 
' d. When depressing or releasing push buttons, 
the mechanism to jar condenser ^^ if equipm ent is 

e. Presetting cannot be accomplished correctly u. 

...not in proper working order. j r push buttons while 

f. Do not depress or release any J^ c " e * £ T0 _ TUNE button 
the mechanism is unlocked unless the PUSHTU iu 

is depressed. 

-:=ri=rur:rrSe 5tf r. 

quency to be preset. T0 . TUNE button and rotate the re- 

b. Hold down the PUSH-iu iu« w hes t frequency to be 
ceiver TUNING control manually to the highest ireq 



preset. 



117 



CEiv E TTUN E RECEIVER TUN E-OPERATE S wi, ch lo RE . 
™i£%GmVZ™ZY™ e '"""""W ""til the receiver 

^tl^Svt'cAras? r ro1 tor max "™ m 

when adjustment is completed GNAL tam P musl 'till glow 

'• Hold down the PUSII-TO-TITlw i,„f. j , 
re S po„di„ B receiver push button a " d depress lhe COT " 

wlS. fhttan"™ TUNING COnlrcl ■*»» "r '0 zero beat 

PUSH-T^UNeZLT PUSh bU "° n ™ W,e "»* W-% the 

of'froScy" '" h indUSiVC f ° r CaCh Chan " cl <» d «^asi nir order 

«o lock potion .STS"" an< ' ~«"» I— tatton assemblies 
an!i S METeScMonTc ^^ CUU ™ teh to ANT «* 

for maximum m;te r re 1 £ I « ( r rCSI ? niiine antenna w "™<™ 

adj„ ,, the antenna eoupL forSntfZ^* "' *"" 
«• Readjust an , ra „ smIUor antcma ^^ ■»• 

to RECErVE,' r TUNF^smo C „ EIV , EI ? ™ NE -°™RAT E switch 
""ttons for ,1™ , 3 ' °e"e™e r "ZZr'T"^ ""* 
any receiver push button „i!T„ i . te 2ero beat - Rc 'l"i« 

second. '* " bcal note over 1000 cycles per 

to OPIATE. 1 ™™'"" RECE,VE R TUNE-OPERATE switch 

s. Turn the receiver TUNE-OPVP atit« « •* i_ , 



MOOULATO* 




RAOIO 


RECEIVER 6C 663 


tAPA^iTORS 


C i.i 


16 uuf MAX 


CI. 3 16 UUF MAX 


CI J l« UUF MAX 


CI. 7 


16 UUF MAX 


CI.2 


62 UUF MAX 


CI.4 


62 UUP MAX 


CI fl 


62 UUF MAX 


ci. a 


62 UUF MAX 


C2 


0.OO6 UF 300 V 


Cl 


0.006 UF 300 V 


C4 


0.006 UF 300 V 


c % 


0.0O6 UF 300 V 


ce 


0.0O6 UF 300 V 


C7 


0.O06 UF 300 V 


ce 


SOO UUF 500 V 


C9 


0. 0O6 UF 300 V 


CtO 


0.75 UUF 500 V 


Cll 


0O6 UF 300 V 


CI2 


0.01 UF 300 V 


Cl3 


SOO UUF SOO V 


CI4 


0.0O6 UF 300 V 


CIS 


0.5 UF eoo V 


CIA 


2 UF 600 V 


CI7 


0.01 UF 300 V 


cie 


0.01 UF 300 V 


Cl« 


0.O02 UF 500 V 


C20. 


o.i uf eoo v 


C2O2 0.I UF «00 V 


C20.3 O.I UF eoov 


C2I 


0.01 UF 300 V 


C22 


0.005 UF 3O0 V 


C23 


40 UF ?5 V 


C24 


i uf eoo v 


C2S 


2 UF eoo V 


C26 


o.ooi ur 500 v 


C3S 


C.006 UF 300 V 


CM 


10 UUF 500 V 


CM 


3 UUF 500 V 


C39 


0.006 UF 300 V 


C4I 


TOO UUF SOO V 


C42 


200 UUF SOO V 


C4J 


IOO UUF 500 V 


C44 


30 UUF iOO V 


C45 


IOO UUF 5O0 V 


C4« 


20 UUF SOO V 


Cftl 


IOO UUF 500 V 


C32 


O.OOI UF 500 V 


C33 


30 UUF SOO V 



C34 
C 35 

C 36 

C57 

C6I 

C62 

C63 

C64 

C65 

C66 

C67 

C7I 

C 72 

C73 

C74 

C 73 

C76 

C77 

C6I 

C62 

C63 

C64 

C63 

C66 

C67 

cee 

c«i 

C92 
C93 
C94 



10 UUF SOO V 
10 UUF 300 V 
30 UU>- SOU V 
0.006 UF 300 V 
IOO UUF 500 V 
0.001 UF SOO V 
30 UUF SOO V 
IO UUF 500 V 
10 UUF 500 V 
30 UUF SOO V 
0.006 UF 300 V 
IOO UUF SOO V 
001 UF 500 V 
30 UUF SOO V 
10 UUF SOO V 
10 UUF 500 V 
30 UUF SOO V 
0.0O6 UF 300 V 
230 UUF *00 V 
006 UF 300 V 
0.006 UF 3O0 V 
3 UUF SOO V 
30 UUF 500 V 
SO UUF 500 V 
23 UUF SOO V 
10 UUF SOO V 
30 UUF SOO V 
30 UUF SOO V 
IOO UUF 500 V 
SO UUF SOO V 



SWITCHES 



I 
02 
03 
04 
03 



REC ON- OFF 
RADIO* EXT* E*T ONLY 
ON- OFF SPEAKER 
ON -OFF S«U(LCH 
TUN! -OPERATE 



LAMP 
£ I CALL SIGNAL (*EOn) 



^, 



USE IS AMP 

JACKS 
J I PHONES 
J 2 PHONES 
J 3 FRONT PANEL JACK 



L I LIMITER CATHODE CHOKE 
L32 ANTENNA COUPLING 

L33 RF plate 

L 34 MOD GRlO 
L4I RF OSCILLATOR 
L42 OSC SHUNT FEEO 
L3I 1ST IF GRIO 
L32 MODULATOR PLATE 
L6I 2N0 IF GRID 
L62 1ST IF PLATE 
L7l LIMITER GRIO 
L72 2ND IF PLATE 
L 61 DETECTOR INPUT 
L62 LIMITER PLATE 
L9I IF OSCILLATOR 



ZLUG3 
PCI " RECEIVER PLUG 
PG2 DVNAMOTOR PLUG 
PC 3 FRONT PANEL PLUG 



TORS 

20,000 ^l i/2W 
230.0OOA 1/2 W 
300 /v 1/2 w 
3C000 -n. 1/2 W 
3,000 a | vv 
IOO. 000 A(^* 
30,000 1 I W 
20.000 A 2 W 
1,000 AI/JW 
230.OO0 sv |/2 * 
I.OOO.OOO A 1/2 W 
2JDOO A 1/2 W 
IO.OOO /X 1/2 W 
230.000 A 1/2 W 
1,000.000 ^ 1/2 W 
1,000,000 /V 1/2 VY 
250,000 *■ 1/2 W 
lOO.OOO /V 1/2 W 
2,000.000 A |/2 * 




R 20 300 ^ I W 
R 22 30OOO A|/2 W 
R 23 3,000 -^ 1/2 W 
R 24 SO /v 1/2 w 
R 23 2.SOO a 1/2 w 
R 2t 1200 A|/2 W 
2,300 Ai/jk 
2.300 A|/- w 
13,000 Ajw 

6,600 -^ I W 
30.000 A 1/2 W 
30,000 A 1/2 W 
R 37 250,000 A (/2 W 
« 36 1.000 A|/i* 

lOOOOO A |/2 W 
• 00.000 A \/i w 
43,000 A|/2W 
100,000 /V 1/2 W 
lOO.OOO ^ 1/2 W 
lOO.OOO A I/2M 
lOO.OOO A 1/2 W 
43,000 A|/J W 
10,000 A 1/2 W 
I.OOO A 1/2 W 
lOO.OOO A|/2 W 
43,000 A 1/2 W 
10,000 ^ 1/2 W 

to.ooo -a r/2 w 
70,000 ^ r/2 w 

250,000 A 1/2 w 
TO.OOO /V 1/2 W 
23O.0O0 A|/iw 
I.OOO A 1/2 W 
lOO.OOO A|^« 
40.000 /V i/2 W 
40.000 A 1/2 W 



R27 
R26 
R29 
R30 
R3I 
«32 
R33 



R4 

RSI 

« 32 

R33 

R54 

R33 

R6l 

R62 

R63 

R64 

R7| 

R72 

R73 

R74 

R6I 

R62 

R63 

R64 

R6S 

R9I 

R92 

R93 



PYNAMQTQR QM-: 
CAPACITORS 
C 701 O.OOS UF 600 V 
C 702 O.OOS UF 6O0 V 
C 703 O.OOS UF 600 V 
C T04 O.OOS UF 600 V 
C 70S' O.OOS uf eoo V 
C 706 0.003 Of eoo V 

COILS 

L 701 HV AF FILTER 

L 702 LV K«* FILTER 

L 703 HV RF FILTER 

L 704 HV RF FILTER 



JACK 
J 701 



RIO 

Rll 
RI2 

Ri3 

RI4 

RIS 

RI6 
R 17 

Rie 

RI9 



TRANSFORMER 
T I OUTPUT TRANSFORMER 

VACUUM TUBES 

VI VT 112 (6AC7) 

V2 VT-II2 (CAC7) 

V3 VT-94 (6J3) 

V4 VT-209 (I25G7) 

V3 VT-209 (I2SG7) 

V6 VT-II2 (6AC7) 



^l DVNAMOTOR JACK 

DYNAMOTQR 
MG 701 12 V DVNAMOTOR 

DYNAMOTQR DM 36^ 124 y 
CAPAC.TQRS 

c eoi o.oos uf eoo v 

ceo2 o oos uf eoov 

ceo3 o.oos uf eoov 

ceo4 o.oos uf eoov 

ceos o.oos uf eoov 

ceoe o.oos ur eoov 

COILS 
L60I HV Af FILTER 

L602 lv rf filter 

L803 HV RF FILTER 
L604 HV RF FILTER 

J*CK 
J 601 DVNAMOTOR JACK 

DYNAMOTQR 
MG60I 24 V DVNAMOTOR 



ve ve vj v , 




VT V9 V4 V3 ^*!° O O O O 






'_! _L !_'_* J - L-'JS^ •« <» •• .t ,. I 



3 9 M_JS J9_l4.i* Jt,!^ 

TiH^-i+r-fja. 




^X °rNAMO T OR DM-34-D 

^ <2 VOLT 



—J Ll 




FIGURE 52. BC-683 Receiver-Schematic Diagram. Extract From Fig. 



4, TMII-620 



driver of insulating material through a hole in the left side of the 
receiver dust co^ 
u. Tune the ol 
mitter changes. 



receiver dust cover. 

u. Tune the other receiver as above, but do not make any trans- 



SECTION III 

RECEIVER ALIGNMENT 

218 GENERAL. Alignment of the SCR-608 receiver should not 
be necessary each time channels are preset. However alignment 
should be checked each time a set is inspected and adjustments 
made wherever the inspection indicates that they are necessary. 
In succeeding paragraphs a method is outlined using an SCR-608 
transmitter to provide a signal of correct frequency when a stand- 
ard signal source is not available. 

219. IF ALIGNMENT. 

a. Mount a good SCR-608 transmitter on Mounting FT-237, 
remove the top plate and set up the crystal for chanel 318. The 
fundamental frequency for this crystal is 441.666 kc and its 
sixth harmonic is 2.65 mc which corresponds to the correct IF 
of the receiver. 

b. Remove the cover from the receiver to be aligned and mount 
the receiver on Mounting FT-237. 

c. Set up a VTVM in accordance with its instructions for meas- 
uring negative DC voltages (30 volt range) and connect its DC 
volts probe to the receiver limiter grid (prong 4 of V6) and its 
common lead to ground. 

d. Ground the grid of the receiver RF oscillator stage (prong 

5 of V3). 

e. Remove the transmitter rectifier tube (V102) from its socket 
and connect the rectifier grid (prong 5 V102) to the receiver 
mixer grid (prong 4 of V2) through a series condenser. If a con- 
denser is not available, use two insulated wires and twist together 
to obtain the effect of a condenser. 

f. Turn the receiver power switch ON, SQUELCH switch OFF, 
and TUNE-OPERATE switch to OPERATE. 

g. Turn the transmitter power switch ON, depress the push 
button for channel 318, and turn the RECEIVER TUNE-OP- 
ERATE switch to RECEIVER TUNE. 

h. After the receiver has warmed up, adjust the primary and 
secondary adjustment of FL3A, FL2A, and FL1A, in turn, for 
maximum meter reading. 

119 



*• Repeat h above two or three times, if necessary, to obtain 
maxi mum meter reading. 

. J- If the meter reading exceeds 18 volts, the tuning of the stage 
will be quite broad. The set's response can be lowered by turning 
tne SQUELCH switch ON and adjusting the SENSITIVITY 
control. However, do not allow the response to fall below 6 volts. 

220. DISCRIMINATOR ALIGNMENT. 

a. Set up the receiver to be aligned and a good transmitter as in 
Paragraph 219. 

b. Connect a VTVM to measure negative voltage at terminal 
No. 8 of FL4 with respect to terminal No. 7 (30 volt range). 

c ; Adjust the primary of FL4 for maximum meter reading. 
rh ls should be about 20 volts. Turn receiver OFF. 

d. Adjust the VTVM to false zero at 1.2 volts on 3 volt range 
(pan 21c). Connect the VTVM to measure negative voltage at 
terminal No. 3 of FL4, with respect to terminal No. 7. 

e. Turn the receiver ON, allow time to warm up, and adjust 
the secondary of FL4 until false zero is obtained when the align- 
ment tool is removed. Vary the secondary adjustment above and 
below resonance and note that the meter reading changes in both 
directions from false zero. 

221. IF OSCILLATOR ALIGNMENT. After the alignment of the 
discriminator is completed, turn the TUNE-OPERATE switch 
to TUNE and adjust the trimmer of LCU4 to obtain zero beat. 

222. RF ALIGNMENT. 

a. Mount a good SCR-608 transmitter on Mounting FT-237 
and set up two crystals for channels near the high and low end 
of the band. 

b. Remove the cover from the receiver to be aligned and mount 
the receiver on Mounting FT-237. 

c. Set up a VTVM in accordance with its instructions for meas- 
uring negative DC voltages (30 volt range), and connect its DC 
volts probe to the receiver limiter grid (prong 4 of V6) and its * 
common lead to ground. 

d. Press the receiver PUSH-TO-TUNE button and rotate the 
TUNING control manually to the channel corresponding to the 
transmitter crystal near the high end of the band. Turn the re- 
ceiver ON. 

e. Depress the transmitter push button for the channel near 
the high end of the band, turn the RECEIVER-TUNE-OP- 
ERATE switch to RECEIVER TUNE. 

120 



f. Adjust the receiver RF oscillator trimmer (C1.7) and then 
the RF amplifier trimmers (C1.4 and C1.5) for maximum meter 

reading. . ■ , ,, 

g. Tune the receiver and transmitter to the channel near the 
low end of the band as in d and e above. 

h. Adjust LCU2A and LCU1A for maximum ^/reading. 
Adjust LCU3A only if tracking is off on the low end of the band. 

i. Repeat d to h above until meter readings are stabilized. If 
meter readings exceed 18 volts, the tuning will be quite broad 
and false peaks may be obtained. To reduce readings, operate 
the receiver from a separate mounting somewhat removed from 
the transmitter. ' 

j. After adjustments are completed, note that zero beat is ob- 
tained when the TUNE-OPERATE switch is turned to TUNE. 

SECTION IV 
TRANSMITTER ALIGNMENT 

*>23 GENERAL. Alignment of the SCR-608 transmitter should 
not be necessary each time channels are preset. However, align- 
ment should be checked each time a set is inspected and adjust- 
ments made whenever the inspection indicates that they are nec- 
essary. 

224. PRECAUTIONS. 

a. Operate the transmitter dynamotor for short periods only. 

b. Do not touch high voltage circuits, for voltages present are 
dangerous. 

225. OSCILLATOR AND 1st AMPLIFIER ALIGNMENT. 

a. Mount the transmitter to be aligned on Mounting FT-237, 
remove the top plate, and unlock the push button selector mecha- 
nism. 

b. Set the METER switch to No. 2 and the TUNE-ANT CUR 
switch to"' TUNE. 

c. Set the condenser C107 to minimum capacity. Open end of 
slot should be to the left. 

d. Turn the power switch ON and the RECEIVER TUNE-OP- 
ERATE switch to RECEIVER TUNE. 

e. Depress the push button for a channel near the high end of 
the band and tune the ganged tuning condensers across the entire 

121 



band. A meter reading should be obtained in all positions of the 
ganged tuning condensers. 

!>/V DePreSS the PUSh button for a chann el near the low end of 
the band and tune the ganged tuning condensers across the entire 
band. A meter reading should be obtained in all positions of the 
ganged tuning condensers. 

g. Turn the METER switch to No. 3, depress the push button 
lor the channel near the high end of the band, and tune the 
ganged tuning condensers for maximum meter reading. 

h. Depress the push button for the channel near the low end 
of the band, and tune the ganged tuning condensers for maxi- 
mum meter reading. 

i- Release the push buttons and lock the selector mechanism. 

226. RECTIFIER ALIGNMENT. 

a. Turn the METER switch to No. 4. 

b. Adjust the tuning slugs in the coils L118 and L119 to ap- 
proximately one inch from the end of the coils. 

c. Set the condensers C153 and C157- to mid-capacity. Open 
end of slot should be to the rear. 

d. Depress the push button for the channel near the high end 
of the band and adjust C153 and C157 for maximum meter read- 
ing on the 12th harmonic of the fundamental crystal frequency 
or one sixth of the output frequency. The correct harmonic may 
be distinguished by use of an absorption type wavemeter of the 
proper range. 

e. Depress the push button for the channel near the low end 
of the band and adjust the tuning slugs in L118 and L119 for 
maximum meter reading. 

f. Repeat d and e above until the meter reads about 35 for d 
and 30 for e. 

227. TRIPLER ALIGNMENT. 

a. Turn the METER switch to No. 1. 

b. Set the condensers C114 and C116 to mid-capacity and the 
tuning slugs in coils L106 and L107 to approximately one inch 
from the end of the coils. 

c Depress the push button for the channel near the high end 
of the band and adjust C114 and C116 for maximum meter read- 
ing. 

d. Depress the push button for the channel near the low end 
of the band and adjust the tuning slugs in coils L106 and L107 
for maximum meter reading. 

122 



e. Repeat c and d above until the meter readings stabilize. 

228. DOUBLER ALIGNMENT. 

a. Turn the METER switch to No. 5. 

b. Set the condenser C120 to mid-capacity and the 1 tuning ^s lugs 
in coil L108 to approximately one inch from the end of the coils. 

c. Depress the push button for the channel near the high end 
of the band and adjust C120 for maximum meter reading. 

d. Depress the push button for the channel near the low end of 
the band and adjust the tuning slug in L108 for maximum meter 
reading. . . 

e. Repeat c and d above until the meter readings stabilize. 

229. POWER AMPLIFIER ALIGNMENT. 

a. Turn the METER switch to No. 6 and turn the RECEIVE 
TUNE-OPERATE switch to OPERATE. 

b. Reduce the antenna coupling to minimum, set the antenna 
trimmer condenser corresponding to the channel near the high 
end of the band to maximum capacity (white dot up) and set the 
antenna trimmer condenser corresponding to the channel near 
the low end of the band to minimum capacity (white dot down). 

c. Depress the push button for the channel near the high end 
of the band, press the microphone switch, and adjust condenser 
C126 for minimum meter reading. 

d. Depress the push button for the channel near the low end 
of the band, press the microphone switch and adjust coil L110 
for minimum meter reading. Coil L110 can be adjusted by using 
a screwdriver to move the toothed wheel at the bottom of the 

coil. ... 

e. Repeat c and d above until meter readings stabilize. 

SECTION V 

SERVICING 

230. GENERAL. Radio Sets SCR-608 and SCR-628 should be 
inspected once each month, or after 300 hours of operation, or 
whenever defective performance occurs, whichever is earliest. 
This inspection must include all auxiliary equipment, spare parts, 
and vehicular installations. In addition to this inspection, every 
opportunity should be taken to check the condition of the set when 
presetting channels. An historical record of all inspections and 
all maintenance work should be kept. 

123 



231. PRELIMINARY INSPECTION. 

a. Obtain a report on performance. 

b. Vehicular Inspection. Inspect the vehicular installation and 
perform the following operations: 

(1) Inspect the vehicle storage battery and note loose or 
corroded connections. Check the state of charge of the battery. 

(2) Inspect the power supply cable from the vehicle igni- 
tion system to the terminal box. Connections must be tight, the 
cable insulation must not be damaged, and the cable must be 
adequately anchored to the body of the vehicle. 

(3) Inspect the power supply cable from the terminal box 
to Mounting FT-237. Connections must be tight, the cable insu- 
lation must not be damaged, proper polarity must be observed, 
and the cable must be adequately anchored and protected where 
it enters Mounting FT-237. 

(4) Inspect the mounting frame and cabinet for physical 
damage and make sure it is mounted firmly to the vehicle body. 

(5) Inspect Mounting FT-237.. It should be well grounded 
to the mounting frame and cabinet, the fuze should be good, a 
spare fuze should be provided, antenna terminals should be un- 
damaged, and cannon sockets should be free from defects. 

(6) Inspect the antenna system. Three sections of antenna 
must be used. If the lead-in wire is more than 40 inches in length, 
coaxial cable should be used. Connections must be firm and not 
corroded or rusted. The mast base must be firmly mounted on the 
vehicle so that the antenna may assume a vertical position, and 
insulator must not be painted. Wire must not be used to tie down 
the antenna. If a coaxial cable feed system is not used, the co- 
axial lead should be removed from the mast base. 

(7) Measure the voltage at the fuze in Mounting FT-237. 
For proper radio operation this should not exceed 14 volts with 
respect to ground with the radio set operating and the vehicle 
engine running at a speed equivalent to 30 miles per hour on 
level highway. 

c. Receivers. Inspect both receiver units and perform the fol- 
lowing operations: 

(1) Inspect and clean cannon plugs, check for play, and 
burnish if necessary. 

(2) Inspect receiver front panel switches, controls, jacks, 
and binding posts for physical damage. 

(3) Remove the receiver dust cover and inspect the chassis 
for surface defects, such as: loose or misplaced wires; damaged 

124 



DKW Lfl7 OSCLLATOR 
~ "~^ VI07 




RADIO TRANSMITTER BC-664-A 

C,APACJTpR,& 
C 101 0.003 UF 500 V 
100 UUF 600 V 
500 UUF 800 V 
15 UUF 500 V 

350 UUF MAX-AIR CAP 0.025* 
60 UUF MAX-AIR CAP 0.025* 
60 UUF MAX -AIR CAP 0.025" 
60 UUF MAX- AIR CAP 0.025" 
60 UUF MAX -AIR CAP 0.025" 
60 UUF MAX -AIR CAP 0.025" 
20 UUF MAX -AIR CAP 0.025" 
500 UUF 600 V 
0.003 UF 500 V 
0.01 UF 300 V 
0.003 UF 600 V 
0.003 UF 500 V 
20 UUF MAX- AIR CAP 0.025" 
20 UUF MAX- AIR CAP 0.025* 
0.003 UF 500 V 
0.003 UF 600 V 
20 UUF MAX- AIR CAP 0.025" 
0.003 UF 500 V 
0.003 UF 500 V 
0.01 UF 300 V 
0.002 UF 1200 V 
60 UUF MAX- AIR CAP 0.030" 
20 UUF MAX- AIR CAP 0.025* 
100 UUF MAX- AIR CAP 0.0195" 
100 UUF MAX- AIR CAP 0.0195" 
100 UUF MAX- AIR CAP 0.0195' 
100 UUF MAX- AIR CAP 0.0195" 
100 UUF MAX- AIR CAP 0.0195" 
100 UUF MAX -AIR CAP 0.0195' 
100 UUF MAX- AIR CAP 0.0195* 
100 UUF MAX -AIR CAP 0.0195" 
IOO UUF M/\X-AIR CAP 0.0195* 
IO0UUF MAX- AIR CAP 0.0195" 
0.5 \Jf 600 V 

C 136.1 0.1 UF 600 V 

CI362 0.1 UF 600 V 

CI39 0.5 UF 600 V 

C 140 30 UF JO V . • 

CI4I 0.5 UF 600 V 

CI42J 0.1 UF 600 V 
- C 142.2 0.1 UF 600 V 

CI43 05 UF 600 V 

2 UF 1000 V 
0.003 UF 500 V 
175 UUF 500 V 
0.003 UF 500 V 
0.001 UF 1200 V 
0.001 UF 500 V 
50 UUF SOO V ' 
20 UUF MAX -AIR CAP 0.025* 
0.003 UF 500 V 
0.003 UF 500 V 
20 UUF MAX -AIR CAP 0.025" 

0003 UF 500 V 
0.01 UF 300 V 



CI03 
CI04 
CI05 

(CI06 
CII3 
CU5 
CII9 
CI52 
C»56 
CI07 
C 106 
CI09 
CIIO 
Clll 
CH2 
CII4 
C 1 16 
CH7 

cue 

CI20 
CI2I 
CI22 
CI23 
Cl24 
CI25 
CI26 
CI27 
CI26 
CI29 
CI30 
CI3I 
CI32 
CI33 
CI34 
CI35 
CI36 
CI37 



CU5 
CH6 
CI47 
C.H6 
CU9 
CI50 
CI5I 
CI53 
CI54 
CI55 
CI57 
CI56 
CI59 




APPARATUS LEGEND 



CI6I 100 UUF 500 V 
C "62 4 UF 50 V 



push button assembly 
crystal selectors 

push button assembly 
ant cap selectors 
ant cur -tune 
receiver tune -operate 
on -off power supply 
meter switch 



PILOT LAMP 
t/2 AMPERE FUSE 



MACNETIC MICROPHONE 
CARBON MICROPHONE 



LIOI 

LI02 

LI03A 

LI04 

LI05A 

LI06 

LI07 

LI06 

LIIO 

Lltl 

LII7 

LII6 

LII9 

LI20 

LI2I 

LI22 
METER , 

MIOI ANT CUR- TUNE METER 

PQTENTiQMETf.o 

P 101 50.000 ^ SIDETONE CONTROL 

PLUGS 

p C 101 TRANSMITTER 

PC 103 OYNAMOTOR 

p C 104 OYNAMOTOR 

RESlSTpqfi 
R 10) 100,000 a |/2 W 

* <02 1,000 a 20 W 
" '03 250,000 ^ | w 

d !S1 700o ° A| /*w 

H ,0 *» 50,000 ■«■ 1/2 w 



1ST RF PLATE CHOKE 
OSC PLATE 

1ST RF PLATE TUNINC 
MODULATION 
RF CHOKE 

TRIPLER PLATE TuNINC 
DOUBLERCRiD TUNING 
OOUBLER PLATE TUNINC 
POWER AMP TUNINC 
ANTENNA COUPLING 
OSC CRID FILTER 
TRIPLER CRID TUNINC 
RECTIFIER PLATE TuNINC 
POWER AMP RF CHOKE 
POWER AMP RF GRID CHOKE 
POWER AMP ANTI-SING 



RI06 

R 107 

RI08 

RI09 

R 110 

Rill 

R If 2 

RII3 

R I 14 

RII5 

R 1 16 

RM7 

RM6 

R 1 19 

RI20 

RI2I 

R 122 

RI23 

R 124 

R 125 

R 126 

R 127 

R 126 

R (29 

R 130 

R 131 

R 132 

R 133 

R 134 

R 135 

R 136 

R 17*7 

R 136 

R 139 

R 140 

R 141 

R 142 

R 143 

R 144 

R 145 

R 147 

R 149 

R 151 

R 153 

R 154 

R 155 

R 156 

R 157 

R 156 



1.000 /v 1/2 W 
30,000 ^ 1/2 W 
30 /v 1/2 W 
75.000 -o- 2 W 
1.000 ■«■ |/2 W 
15,000 /v ) w 
10 /v |/2 w 
1.0 a iow 
30.000 -A. 2 W 
100,000 a |/2 w 
30,000 A2w 
50,000 ■«■ 1/2 W 
30,000 /v 2 W 
1.200 a 1/2 W 
200 ■«. I w 
100 ^ 1/2 W 
250,000 t I W 
10.000 ■". I W 
10.000 Aiyjw 
5.000 ^ 1/2 W 
75,000 A2w 
t.O a IOW 
10,000 *>> 1/2 W 
5,000 /v 25 W 
100 aiow 

•2 «■ 25 W 

12 ^ 25 w 

60 ^ 10 W 

30.000 A. i/z w 

30 /v 10 W 

30.000 A|/2 W 

100 -n. |/2 w 

50.000 ^ 1/2 w 

100,000 A|/2W 

»2 -* 1 25 W 

12 ^ 25 W 

180,000 ■«■ I W 

100 /v !/ 2 W 

100.000 *> i/2 W 

50.000 A^w 

1,000 a jow 

50 ^ 1/2 W 

100,000 r< | w 

*.3 --t |79 w 

»3 A. (7,9 w 

100.000 ** 1/2 W 

30A | W 

300 /x | w 

50 *v |/2 w 



THERMOCOUPLE 
TC 101 ANT CUR THERMOCOUPLE 

THERMOSTAT 
TO 101 OVEN THERMOSTAT 



RELAYS 

S 101 ANTENNA SWITCHING 

5 102 OYNAMOTOR STARTING 

3 103 RECEIVER DISABLING 

TRANSFORMER^ 

T 101 AF INPUT 

T 102 af output 



VACUUM TUBES 


VIOI 


VT- 164 f)6l9) 
VT-164 (16(9) 


VI02 


VI03 


VT-164 (1619) 


VI04 


VT 165 (1624) 


VI05 


VT- 164 (1619) 


VI06 


VT- 164 (16'*) 


VI07 


VT-164 (1619) 


VI06 


VTI64 (1619) 


CRYSTAL HOLDERS 


TO 

yiio; 


CRYSTAL HOLDERS 


FT-24I-D 



OYNAMOTOR DM-3Vr> i ? » 

CAPACITORS 

C 501 0.003 UF 800 V 

C 502 0.003 UF 800 V 

C503 0.003 UF 600 V 

JACKS 
J 501 OYNAMOTOR 
J 502 DYNAMOTOR 

OYNAMOTOR Q M -37-( ) ? * y 

CAPACITORS lr ~ ! - 

C 601 .003 UF 800 V 

C602 0.0O3 UF 800 V 

C603 003 UF 800 V 

JACKS 
J 601 DYNAMOTOR 
J 602 DYNAMOTOR 




M*2 RECEIVER (RF) 
BATTERY (+) 
j-« j a GROUND (-BATT) 



L__°4_4e__4^ 4 3 A 5 6 7 6 Z , 

PC 103 " PG 104 

jJSOl DM- 35- D 12 VOLT OYNAMOTOR J 502 



.."!S1 0M-37JJ24J^TDYNAJU0T0R J 602 



I 1- , ' . « ' *- tv«_. utnaMOTOH J602 

l . i r : ■ ■ 1 n i~ K 

c *Yt\ r._\-... i + ! c «./^r\ !/t\ 




FIGURE 53. 



BC-684 Transmitter- Schematic Diagram. Extract From Fig. 6, TMM-620 



... .j 



filter units; damaged tuning units; damaged variable condensers; 
defective or missing fuze or spare; defective switch contacts; 
defective tube socket contacts; incorrect tube types or tubes im- 
properly inserted in the sockets; defective or missing shields; 
bent or broken parts ; or unusual odors. 

(4) Clean chassis and parts. 

(5) Clean push button assembly with dry, clean air (do not 
use carbon tetrachloride) . Lubricate as specified in the technical 
manual, if required. Do not allow lubricant to get on the locking 
wedge. 

(6) Remove the dynamotor and inspect plug connections, 
brushes, brush holders, commutators, and bearings. 

(7) Replace brushes, clean commutator, and lubricate bear- 
ings as specified in the technical manual, if necessary. Do not 
allow lubricant to get on the commutator or brushes. Clean 
thoroughly with dry, clean air. 

d. Transmitter. Inspect the transmitter unit and perform the 
following operations: 

(1) Inspect and clean cannon plugs, check for play, and 
burnish if necessary. '...'■ 

(2) Inspect transmitter front panel and right end panel 
switches, controls, meter, jacks, and binding posts for physical 
damage. 

(3) Check crystal compartment and spare crystal tray. 

(4) Remove the dust cover and inspect chassis for surface 
defects, such as: loose or misplaced wires; damaged coils; loose, 
shorted, or damaged trimmer and tuning condensers; defective 
or missing fuze and spare; defective switch contacts; defective 
tube socket contacts ; incorrect tube types or tubes improperly 
inserted in the sockets; defective or missing shields; bent or 
broken parts ; or unusual odors. 

(5) Clean chassis and parts. 

(6) Clean the push button assembly with clean, dry air (do 
not use carbon tetrachloride). Lubricate as specified in the 
technical manual, if required. 

(7) Remove the dynamotor and inspect and treat as in c 
above. 

(8) Inspect relays, and burnish points when necessary. Use 
burnishing tool only. Do not use abrasive papers or cloth. Make 
sure that the points close when the relay operates. 

e. Other equipment. Inspect microphone, headset, remote con- 
trol unit, and all other auxiliary equipment or spare parts for 

125 



evidence of physical damage or poor general condition. Clean 
when necessary and replace defective or missing parts. 
* f. Operation. With the transmitter and receivers mounted in 
the vehicle and connected to the antenna and power supply with 
which it is to be used, perform the following operations: 

(1) Turn the receiver power switch ON. Dynamotor should 
start. 

(2) Turn the SPEAKER switch ON and turn the VOLUME 
control to its maximum clockwise position. After 20-30 seconds, 
operation should be indicated by noise from the speaker when 
the SQUELCH switch is OFF. The CALL SIGNAL LAMP 
should light. 

(3) Turn the SQUELCH switch ON and extinguish lamp 
with the SENSITIVITY control. Noise from the speaker should 
cease. 

(4) Plug in the microphone and turn the transmitter power 
switch ON. Indicator lamp 'should light. 

(5) Depress a transmitter push button, turn the RECEIV- 
ER TUNE-OPERATE switch to OPERATE, and press the mi- 
crophone switch. Dynamotor should start and meter should in- 
dicate antenna current. 

(6) Check the push button set up by depressing corre- 
sponding buttons on receiver and transmitter, turning the re- 
ceiver TUNE-OPERATE switch to TUNE, pressing the micro- 
phone switch and noting a beat note under 1000 cycles per sec- 
ond on each channel. 

(7) Turn the OUTPUT-TO-PHONES switch ON, TUNE- 
OPERATE switch to OPERATE, and SPEAKER switch OFF, 
and check headphone operation.^ 

(8) Turn the TUNE-ANT CUR switch to TUNE and check 
panel meter readings with the transmitter operating on each 



Switch 
Position 


1 


2 


3 


4 


5 


6 


Circuit 


Doubler 

Grid 
Current 


IstRF 

Grid 

Current 


Rect. 

Grid 

Current 


Tripler 

Grid 
Current 


PA 

Grid- 

Current 


Tot. Plate 

& Screen 

Current 


27.0 • 
38.9 


35 
50 


40 
10 


15 
15 


30 
35 


20 
30 


70 
70 



126 



push button for each position of the METER switch. Readings 
obtained should be as indicated in the table below for channels 
270 and 389. Readings for other channels may be obtained by- 
interpolation. 

(9) An incoming signal should be received without distor- 
tion. 

(10) With the microphone switch pressed, an undistorted 
signal should be transmitted. 

g. Alignment. Defective presetting or alignment may not al- 
ways be discovered in these tests, but will be in the final operat- 
ing check. If the receiver is properly aligned, dial readings will 
correspond to the channel marked on the corresponding crystal 
in the transmitter, reception will not be weak or distorted, and 
zero beat of the receiver will be in the exact center of the range 
through which the CALL SIGNAL lamp is lighted. If the trans- 
mitter is properly aligned, peak panel meter readings for METER 
switch positions 3, 4, and 1 will occur at the same setting of the 
gang tuning control for each channel. " If the presetting or align- 
ment is known or suspected to be defective, proceed at once to 
a complete retuning or realignment of the set (par. 214-229). 

h. The preceding steps should indicate whether the set is good 
or defective. If apparently good, proceed directly to the final 
operating check. If symptoms of trouble have been discovered, 
locate and remedy the trouble before performing the final oper- 
ating check. Do not attempt repairs where successful comple- 
tion is uncertain. 

232. COMMON TROUBLES. 

a. General. Common troubles of the SCR-608 are indicated 
below according to the symptoms or indication they produce. 
This is not a complete detailed list and is to be used only in con- 
junction with standard service methods (chapter 9). 

b. Dynamotor does not operate. 

(1) Defective fuze. (Check for shorts in dynamotor and 
tube heater circuits before replacing.) 

(2) -Weak battery or defective battery connection. 

(3) Defective switch or connections. 

(4) Defective dynamotor. 

c. Dead receiver. 

(1) SENSITIVITY control improperly adjusted. 

(2) Defective speaker or headphones. 

(3) Defective tube or tubes. 

(4) Defective dynamotor connections. 

127 



(5) Internal short. (If smoke appears, turn off the re- 
ceiver and check for shorts using an ohmmeter.) 

d. Weak or distorted reception. 

(1) Dynamotor defective. 

(2) Weak battery or defective battery connections. 

(3) Receiver improperly aligned. 

(4) Defective speaker or headphones. 

(5) Defective antenna system. 

(6) Defective antenna relay. (Connect antenna lead-in to 
REC binding post on FT-237 to check relay.) 

(7) Defective cables or connections in Mounting FT-237. 
(Remove receiver from its mounting, connect a power supply 
directly to the connections on the cannon plug, and connect the 
antenna lead-in to "A" binding post of the receiver.) 

e. Transmitter dynamotor runs but pilot lamp does not light. 

(1) Defective lamp. 

f. Pilot lamp lights but dynamotor does not run. 

(1) Weak battery. 

(2) Defective connection ( high resistance contact). 

(3) Defective microphone control circuit. (Check by turn- 
ing RECEIVER TUNE-OPERATE switch to RECEIVER TUNE 
position.) 

(4) Defective dynamotor , relay contacts. 

(5) Defective dynamotor. 

g. Pilot lamp does not light and dynamotor does not run. 

(1) Fuze in FT-237 defective. 

(2) Wires or connections in FT-237 or power circuits de- 
fective. 

(3) Defective battery. 

(4) Defective transmitter power switch. 
h. No antenna current. 

(1) Defective meter thermocouple. 

(2) Defective transmitter fuze. (Check sidetone in head- 
phones — if OK, fuze is good.) 

(3) Defective crystal. 

(4) Defective tube or tubes. 

i. No antenna current and no current in No. 6 METER switch 
position. 

(1) Defective fuze. 

(2) Defective dynamotor. 

j. No antenna current, *4 normal current in No. 6, and no cur- 
rent in No. 2. Defective front bank of tubes. 

128 



k. No antenna current, 2/3 normal current in No. 6, and high 
current in No. 2. Defective rear bank of tubes. 

1. No antenna current, nearly normal current in No. 6, and low 
to normal current in No. 2. 

(1) Circuit improperly aligned. 

(2) Defective crystal. 

(3) Defective tube. 

m. No antenna current and normal current in all meter posi- 
tions. 

(1) Defective thermocouple. 

(2) Defective antenna system. 

n. Lower than normal current in all meter positions. 

(1) Weak battery. 

(2) Weak tube (especially 1st RF amplifier stage). 

(3) Circuits not tracking. 

(4) Defective condensers or resistors. 

o. Low antenna current, low current in No. 6, and weak trans- 
missions. 

(1) Defective power amplifier tube. 

(2) Power amplifier aligned improperly. 
p. High antenna current. 

(1) Defective antenna system. 

(2) Defective thermocouple. 

q. Normal antenna current, but unable to contact other sta- 
tions. 

(1) Defective microphone. 

(2) Defective transmitter audio amplifier (check side- 
tone). 

(3) Defective antenna circuit. 

(4) Transmitter tuned on wrong harmonic. 

r. Peaks obtained when METER switch is at No. 3, 4, and 1 
with different setting of gang tuning control. Transmitter im- 
properly aligned. 

233. FINAL OPERATING CHECK. 

a. General. The final operating check must be a rigid test of 
the set's performance under conditions which will indicate its 
ability to receive and transmit satisfactorily at extreme ranges. 
Two methods are outlined in b and c below for this check. Either 
will be satisfactory if performed by experienced personnel who 
are thoroughly acquainted with the set's specifications and char- 
acteristics. 

b. Readability test. Install the radio set to be tested in the 
manner in which it is to be used. Set up a test set, which is known 

129 



to be good, on the same channels as the set to be tested. Move 
the set to a point fifteen miles from the set being tested and 
with favorable intervening terrain. Have each set transmit, in 
turn, on each of its preset channels and note the reception on 
each receiver. It should be undistorted and loud enough to be 
understood. Squelch circuits should be inoperative during this 
test since they reduce receiver sensitivity at extreme ranges. If 
the intervening terrain is such that it would reduce signal 
strength, good results can not be expected at maximum range. 
Reduce the distance between stations to compensate for this 
variation from standard conditions and repeat the test. 

c. Voltage test. Install the radio set to be tested in the manner 
in which it is to be used. Set up an SCR-608 test set, which is 
known to be good, on the same channels as the set being tested. 
(The test set must be modified as indicated in paragraph 234 so 
that limiter grid voltage may be measured.) Move the test set 
to a point 10-15 miles from the set being tested (with average 
intervening terrain) and continue test as follows: 

(1) Have set being tested transmit its mean frequency and 
tune test set receiver manually to zero beat. 

(2) Measure the limiter grid voltage on the test receiver 
while the set being tested is transmitting. 

(3) Compare the reading obtained in (2) above with read- 
ings obtained under similar conditions with . SCR-608 trans- 
mitters known to be good. This comparison will give a relative 
indication of transmitter power output. Reading should not vary 
more than 50% from standard reading. 

(4) Have the set being tested transmit a modulated signal 
and check for absence of distortion at the test set. 

(5) Move the test set to a point where the noise level in 
the receiver being tested is just strong enough to be heard while 
the test set is transmitting. 

(6) Have the test set transmit a modulated signal and 
check for readability at the receiver being tested. It should be 
readable despite the noise level. 

(7) Repeat (5) and (6) above using the other receiver of 
the set being tested. 

(8) Repeat (1) to (7) above on the other preset channels. 

(9) If the intervening terrain is unfavorable, this fact 
" must be considered in interpreting results. 

234, SCR-608 TEST SET. In order to conduct the final operating 
check in paragraph 233c, one SCR-608 receiver in each organiza- 

130 



tion should be modified to facilitate measuring limiter grid volt- 
age. This modification in no way impairs the efficiency of the 
set for other purposes. Proceed as follows : 

(1) Remove the dust cover from the receiver to be modified. 

(2) Solder one end of the lead wire of a 1-3 megohm re- 
sistor to terminal No. 6 of Filter Unit FL3A (limiter grid). This 
lead wire should be as short as practicable. 

(3) Remove the spare fuze from its receptacle. 

(4) Terminate the other lead wire of the resistor at a ter- 
minal pin or jack which can be mounted in place of the spare 
fuze. It must be insulated from ground. 

(5) Replace dust cover. ,. 

(6) To measure limiter grid voltage insert the DC volts 
probe of a VTVM in the spare fuze receptacle to contact the 
terminal of the limiter grid lead wire, and ground the common 
lead at the TUNE-OPERATE switch. 



131 



CHAPTER 13 
RADIO SET SCR-284 

Paragraphs 

Section I. General 235 

II. Alignment and neutralization 236-241 

III. Servicing 242-247 

SECTION I 
GENERAL 

235. GENERAL. 

a. Radio Set SCR-284 is a transmitting and receiving set cap- 
able of CW or amplitude modulated voice operation. 

b. Details concerning the functioning of parts, method of in- 
stallation and operation, and maintenance data are contained 
in TM 11-275. These notes are intended to supplement TM 11- 
275 and to provide field artillery repairmen with certain items 
of information of value to him in maintaining equipment. 

c. Refer to figures 54 and 55 for identification of all parts 
and test points. Since more than one model of this set is in use, 
these schematic diagrams may differ in details from those which 
accompany any particular set. 

SECTION II 
ALIGNMENT AND NEUTRALIZATION 

236. RECEIVER ALIGNMENT, GENERAL. 

a. Alignment of the SCR-284 receiver should not be necessary 
at frequent intervals. However, alignment should be checked 
each time a set is inspected, and adjustments made whenever the 
inspection indicates that they are necessary. 

b. The self contained crystal oscillator is used in the receiver 
alignment procedures outlined below to obtain signals near the 
high and low end of the band. However, any signal generator 
could be used for this purpose. If the IF stages are badly out of 
alignment, a frequency meter or a good receiver should be used 
as the signal source. 

132 



237. RECEIVER IF AND BFO ALIGNMENT. 

a. Remove the set from its case and connect to a good receiver 
power supply so that the receiver and the crystal oscillator are 
operative. 

b. Connect the bare end of an insulated wire to the antenna 
terminal of the receiver (No. 12 of plug 2-K-l) and couple the 
other end of the wire to the crystal oscillator by wrapping one 
or more turns around the CRYSTAL holder. 

c. Set up a VTVM in accordance with its instructions for meas- 
uring DC voltage and connect it to measure a minus voltage (3 
volt range) at the common terminal of 2-R-12, 2-R-19 and 2-R-18 
with respect to prong 7 of the mixer tube (2-V-2). 

d. Turn the main switch to VOICE and the AVC switch OFF 

e. Pull the CRYSTAL switch OUT. 

f. Tune the receiver to. a crystal check-point near 3800 kc as 
indicated by maximum reading on the VTVM when the receiver 
tuning dial is near 3800. > 

g. Loosen the lock nut on the slotted shaft on the under side 

?u ,f™Jl aSsis a " d adjust coil ^L- 10 for maximum reading on 
the VTVM. 

h. Repeat procedure in g above for coils 2-L-9 2-T 7 
2-L-6, 2-L-5, and 2-L-4, in turn. ' ' 

i. Repeat g and h above. 

j. Turn MAIN switch to CW and adjust coil 2-L-8 to obtain 
zero beat. Refasten all lock nuts. 

238. RECEIVER RF ALIGNMENT. 

a. Make connections indicated in paragraph 237 a to e in 
elusive. 

• ?•' T ? n A the receiver t0 a crystal check-point near 5800 kc as 
indicated by maximum reading on the VTVM when the receiver 
tuning dial is near 5800. 

c. Adjust the oscillator trimmer condenser 2-C-22 and the 
receiver dial until maximum reading on the VTVM is exactly on 
5800 kc. 

the VTVM* the ante " na trimmer 2_C - 2 for ma ximum reading on 
VTVM dJUSt the RF trimmer 2-C-8 for maximum reading on the 

• *,' T ^\ the receiver to a crystal check-point near 3800 kc as 
indicated by maximum reading on the VTVM. 

g. Adjust the antenna coil 2-L-l and then the RF coil 2-L-2 
for maximum reading on the VTVM. A lock nut must be loosened 
and then retightened in each case. 

133 



h. Repeat b, d, e, f, and g. 

i. Retighten all lock nuts carefully and check receiver operation 
over the entire frequency band. 

239. TRANSMITTER OSCILLATOR ALIGNMENT. 

a. Connect the transmitter to a good Power Unit PE-103-A 
using a well charged 6 volt battery. Do not use a 12 volt battery. 

b. Leave the receiver connected to the transmitter, but place 
it to one side. Close switches l-S-5 and l-S-6 on the receiver 
shelf. 

c. Turn the STANDBY switch to LOW and the MAIN switch 
to CW. 

d. Pull the CRYSTAL switch OUT. 

e. Tune the receiver exactly to zero beat with the crystal oscil- 
lator at 5800 kc. 

f. Tune the transmitter to the setting corresponding to 5800 
kc as obtained from the CALIBRATION CHART. 

g. Turn the screwdriver adjustment labeled CALIBRATION 
to obtain zero beat with the crystal oscillator and the receiver. 

240. BUFFER AND POWER AMPLIFIER ALIGNMENT. 

a. Continue as in paragraph 239. 

b. Push the CRYSTAL switch IN. 

c. Hold down the key and vary the ANTENNA COUPLING 
and ANTENNA TUNING controls to obtain maximum reading 
on the ANTENNA CURRENT meter. 

d. Adjust the buffer trimmer condenser l-C-32 for maximum 
reading on the ANTENNA CURRENT meter. 

e. Adjust the power amplifier trimmer condenser l-C-40 for 
maximum reading on the ANTENNA CURRENT meter. 

f. Repeat c, d, and e above. 

241. NEUTRALIZATION. Neutralizing condensers are provided 
to prevent oscillation of transmitter amplifiers; however, this is 
seldom or never necessary. For the procedure in neutralization 
consult TM 11-275. 

SECTION III 

SERVICING 

242. GENERAL. Radio Set SCR-284 should be inspected once 
each month, or after 300 hours of operation, or whenever defec- 
tive performance occurs, whichever is earliest. The inspection 

134 



VT-147 



VT-146 



VT-W6 



VT-223 



VT-221 




*-4 



2C4S 

0.1 nf 



NOTE: 

ALL TUNING ' 
CAPACITORS 
ARE GANGED 



^^ 



■«* ]^ C 2> t* '* 2 1 g ° " 




„- ITsTbI ISIC L&IQ— " 

1 I I 220* ! ISO* t I 






f | AW * I , ^ ^f^^ -<p 1— | ' , 2 3 

f---"-~-"-^f>-T--------H^---- 



111-149 

Immf 

T2-C25 

fc 1*143 
1 ""- 



6 o o ^ <fr o o o 



.-4-Ji*A- 



WIRING AND SWITCHES LOCATED IN TRANSMITTER 
WHICH AFFECT RECEIVER OPERATION SHOWN WITH 
DASHED LINES. 




C.W. BIAS CONTROL 



B.F.O -I.5V. 



BIAS SUPPLY 



A SUPPLY- 1.5V 



|I0 II 12 

RADIO GROUND A + 



^-^PILOI I 



++ 



-J 2 j e 



2-RI7 
220** 



2RMJ ^uh, 22011 
OFF LO d 




/_- 



OfF 



A VC SWITCH 
2 3 I 



M 

PILOT LIGHT SWITCH 
2 32 



FIGURE 54. BC-654 Receiver- Schematic Diagram. Extract From Fig. 42, TMll-275 



must include all auxiliary equipment, spare parts and vehicular 
installations. An historical record of all inspections and all main- 
tenance work should be kept. 

243. PRELIMINARY INSPECTION. 

a. Obtain a report on performance. 

b. Vehicular inspection. Inspect the vehicular installation and 
perform the following operations: 

(1) Inspect the vehicular storage battery and note loose 
or corroded connections. Check the state of charge of the battery. 
* (2) Inspect the power supply cable from the vehicle igni- 
tion system to Power Unit PE-103-A. Connections must be 
tight, the cable insulation must not be damaged, the cable must 
be adequately anchored to the body of the vehicle, and proper 
polarity must be observed. 

(3) Inspect Frame FM-41-A. It should be firmly fastened 
to the body of the vehicle and the shock absorber device must be 
free from defects. 

(4) Remove Power Unit PE-103-A from the mounting 
frame and inspect plug connections, brushes, brush holders, com- 
mutators, bearings, circuit breakers, relay, and switch. 

(5) Replace brushes, clean commutator, dress relay, and 
lubricate bearings as specified in the technical manual if neces- 
sary. 

(6) Inspect the antenna system. Connections must be firm 
and not corroded or rusted. The mast base must be firmly 
mounted on the vehicle so that the antenna may assume a verti- 
cal position, and the mast base insulator must not be painted. 
The antenna must not be tied down with wire, either insulated 
or uninsulated. If a coaxial cable feed system is not used, the 
coaxial lead should be removed from the mast base. 

(7) Measure the voltage at terminal No. 5 of plug 3-K-l 
(PE-103-A). For proper radio operation, this should not ex- 
ceed 7.1 volts for a six volt system or 14 volts for a twelve volt 
system with respect to ground with the vehicle engine running 
at a speed equivalent to 30 miles per hour on level highway. 

(8) Inspect the power cord for damaged insulation and 
defective plugs. Test for continuity if necessary. 

(9) Inspect the Power Converter PE-104-A for surface 
defects. 

c. Receiver and transmitter. Inspect the receiver and transmit- 
ter unit and perform the following operations: 

135 



(1) Inspect the set case for possible damage to fasteners 
and antenna mounting. 

(2) Inspect front panel switches, controls, jacks, meter, 
lights, and binding posts for physical damage and mechanical 
operation. 

(3) Remove the receiver and transmitter unit from its 
case and inspect chassis for surface defects, such as: loose or 
misplaced wires; defective switch contacts or operation; defec- 
tive tube socket contacts; incorrect tube types or tubes improp- 
erly inserted in the sockets; defective or missing spare tubes; 
defective or missing spare panel lamps; defective or missing 
shields; bent or broken parts; defective antenna loading coil; or 
unusual odors. 

(4) Clean chassis and parts. 

(5) Inspect relay and burnish points when necessary. Use 
burnishing tool only. 

d. Other equipment. Inspect microphone, headsets, remote con- 
trol unit, hand generator, counterpoise, guys, antenna insulator, 
and all other auxiliary equipment or spare parts for evidence 
of physical damage or poor general condition. Clean when neces- 
sary and replace defective or missing parts. 

e. Operation. With the radio set mounted in the vehicle and 
connected to the antenna and power supply with which it is to 
be used, perform the following operations: 

(1) Turn the MAIN switch to CW. 

(2) Pull crystal switch out and rotate the receiver TUN- 
ING control and listen for beat notes at all even hundred fre- 
quencies (3800, 4000, 4200, etc.). 

(3) Check operation of manual volume control. 

(4) Turn MAIN switch to VOICE. 

(5) Tune in amplitude modulated signal and check for 
quality of reception. 

(6) Turn AVC switch ON and check operation of auto- 
matic volume control circuit on strong signals. 

(7) Zero beat transmitter to receiver near the high and 
low end of the band (CRYSTAL switch OUT) and check calibra- 
tion of transmitter. 

(8) Load antenna at each end of the band (CRYSTAL 
switch IN). 

(9) Check CW and voice operation in both HIGH and 
LOW positions of the STANDBY switch. 

(10) Substitute ground power supply and antenna and 
repeat (7) to (9), inclusive. 



136 




FIGURE 55. BC-654 Transmitter- Schematic Diagram. Extract From Fig. 4 4, TMII-275 



f. Alignment. Calibration of the transmitter oscillator and the 
receiver may be checked by comparison with the crystal oscilla- 
tor (par. e (2) and (7) above). However, defective alignment of 
circuits may not always be discovered in these tests, but will be 
in the final operating check. If alignment of either receiver or 
transmitter is known or suspected to be defective, proceed at 
once to a complete alignment of the set (par. 236-240). 

g. The preceding steps should indicate whether the set is good 
or defective. If apparently good, proceed directly to the final 
operating check. If symptoms of trouble have been discovered, 
locate and remedy the trouble before performing the final oper- 
ating check. 

244. COMMON TROUBLES. 

a. General. Common troubles of the SCR-284 are indicated be- 
low according to the symptoms or indication they produce. This 
is not a complete or detailed list and is to be used only in con- 
junction with standard service methods (chapter 9). 

b. Dead receiver. 

(1) PE-103-A switch set for 12 volts and should be 6 volts. 

(2) PE-104-A switch set for 12 volts and should be 6 volts. 

(3) Defective vibrator in PE-104-A. 

(4) Defective copper oxide rectifier in PE-104-A (de- 
teriorates in salt laden air). 

(5) Switch l-S-6 (door of battery compartment) open. 

(6) Switch l-S-5 (rear of battery compartment) open. 

(7) Dead storage battery. 

(8) Defective BA-43. 

(9) Defective tube or tubes. 

(10) Receiver improperly aligned. 

c. Weak receiver. 

(1) Defective PE-104-A. 

(2) Weak storage battery. 

(3) Weak BA-43. 

(4)^ Receiver improperly aligned. 

(5) Weak tube or tubes. 

(6) Antenna grounded. 

(7) Headset shorted. 

(8) Counterpoise open. 

d. Receives voice but not CW. 

(1) Defective BFO tube. 

(2) BFO improperly aligned. 

137 



e. Unable to zero beat receiver to crystal oscillator. 

(1) Defective BFO tube. 
* (2) Defective crystal oscillator tube. 

(3) Defective or missing crystal. 

(4) Defective crystal switch. 

f . Receiver howls. Defective contacts at terminals No. 1 and 2 
of 2-K-2. (May be possible to stop howl by grounding No. 4 of 
AF transformer.) 

g. PE-103-A does not run. 

(1) Circuit breakers not ON. 

(2) 6-12 volt switch in wrong position. 

(3) Defective connections to battery. 

(4) Dead storage battery. 

(5) Defective power cord. 

(6) Defective microphone. 

(7) Switch l-S-5 open. 

(8) Defective relay. 

h. Unable to zero beat transmitter to receiver. 

(1) Defective crystal switch. 

(2) Defective transmitter tube. 

(3) Defective power supply. 
i. No antenna current. 

(1) Defective meter. 

(2) Defective PE-103-A or GN-45. 

(3) Defective key leads. 

(4) Defective keying relay. 

(5) Defective tube or tubes. 

(6) Antenna open or grounded. 

(7) Defective contact wheel on antenna tuning coil. 

(8) Defective power cord. 
j. Low antenna current. 

(1) Defective power supply. 

(2) Defective tube or tubes. 

(3) Transmitter improperly aligned. 

(4) Defective antenna system. 
k. Transmits CW but not voice. 

(1) Defective microphone. 

(2) Defective modulator tube. 

1. Transmitter skips dots. Keying relay improperly adjusted. 

245. FINAL OPERATING CHECK. 

a. General. The final operating check must be a rigid test of 
the set's performance under conditions which will indicate its 
138 



ability to receive and transmit satisfactorily with both CW and 
voice at extreme ranges. This method will be satisfactory if per- 
formed by experienced personnel who are thoroughly acquainted 
with the set's specifications and characteristics. 

b. Readability test. Install the radio set to be tested in the 
manner in which it is to be used. Set up a test set, which is known 
to be good, and move it to a point fifteen miles from the set be- 
ing tested and with favorable intervening terrain. Have each 
set transmit with voice, in turn, on frequencies near the high and 
low ends of the band, if possible, or on their assigned frequencies. 
Note the reception on each receiver. It should be undistorted and 
loud enough to be easily understood. Change to CW and repeat 
test. If atmospheric conditions and intervening terrain are such 
that they would reduce signal strength, good results cannot be 
expected at maximum range. Reduce the distance between sta- 
tions to compensate for this variation from standard conditions 
and repeat the test. 

c. Voltage test. To obtain a more precise test of relative trans- 
mitter power output proceed as follows: 

(1) Set up an SCR-284 test set, which is known to be 
good, on the frequency assigned for testing. Preferably two fre- 
quencies should be used, one near the high end of the band and 
the other near the low end of the band. 

(2) Remove the soft rubber grommet from the center of 
the battery compartment door on the test set. Pass two test leads 
through the hole in the door and connect to terminals No. 3 and 
11 of the receiver terminal strip 2-K-l. 

(3) Set up a VTVM in accordance with its operating in- 
structions and connect it to the test leads to measure negative 
rectified voltages at No. 3 with respect to No. 11. This is AVC 
voltage. 

(4) Install the radio set to be tested in the manner in which 
it is to be used. 

(5) Move the test set to a point fifteen miles from the set 
being tested and with favorable intervening terrain. 

(6) Have the set being tested transmit an unmodulated 
signal. 

(7) Tune the receiver of the test set for maximum AVC 
voltage. 

(8) Record the maximum AVC voltage and compare with 
readings obtained under similar conditions with SCR-284 trans- 
mitters known to be good. This comparison will give a relative 

139 



indication of transmitter power output. Readings should not 
vary more than 50% from standard reading. 

■ (9) If the intervening terrain is very unfavorable this 
fact must be considered in interpreting results. 



140 



APPENDIX I 
ABBREVIATIONS 

1. ABBREVIATIONS. Abbreviations commonly used by radio 
technicians and used in this manual are listed below with their 
respective meanings: 

. Audio fre.quency.__ jf__ AF 

Alternating current AC 

Amplitude modulated . AM 

Automatic volume control AVC 

Beat frequency oscillator... 1 BFO 

Continuous wave.. C W 

Delayed automatic volume controL.. DAVC 

Direct current DC 

Fahrenheit™ p 

Frequency modulated , FM 

Intermediate frequency IP 

Kilocycles (per second) kc 

Megacycles (per second) mc 

Microfarad „.. m fd 

Radio frequency RF 

Vacuum tube voltmeter VTVM 



141 



APPENDIX II 
PARTS COLOR CODING 

1. GENERAL. Values of standard radio components may fre- 
quently be identified by a color code system. 

2. RESISTORS. 

a. Numbers. Numbers are represented by the following colors: 

0— black 5— green 

l_brown 6— blue 

2 — red 7— violet 

3— orange 8— gray 

4— yellow 9— white 

b. Body, tip, and dot system. Three colors are used on each re- 
sistor. The body color represents the first figure of the resistance 
value; the tip or end color represents the second figure; and, the 
dot color near the center of the resistor represents the number 
of zeros following the first two figures. For example, a resistor 
with green body, orange tip, and red dot has a value of 5300 ohms. 

TIP DOT BODY BODY TIP DOT 

LUL 



£ 





ORANGE E»RED EMM GREEN 



RESISTANCE VALUE 5300 OHMS 
FIGURE 56. Resistor code. 

c. Band system. Resistors may also be marked by three colored 
bands. The band nearest the end of the resistor is the body color 
the next band is the tip color, and the third band is the dot color' 
A fourth band may be added to indicate tolerance as follows: 

5%— gold 
10% — silver 
20% — no color 
3. CONDENSERS. 

a. Three dot system. Small mica condensers are sometimes 
marked with dots using the same color code as in paragraph 2 
above. Capacity is indicated in micro-micro-farads. The first 
142 



two dots are significant figures and the third dot indicates the 
number of zeros after the second significant figure. An arrow 
or other appropriate symbol indicates the proper direction to 
read. Tolerance is sometimes indicated by a single dot on the 
reverse side of the condenser according to the following code: 

2^2%— white 15%— yellow 

5 Jo— green 20%— red 

10 Jo— blue 



3500 



>y*f 



325,000,* 



TS 



500 VOLT 
6% TOLERANCE 

[GREEN SI 

FIGURE 57. Condenser code. 



IRED 



I BLUE 



b. Six dot system. If more than two significant figures are 
required, a six dot system may be used. The three upper dots 
are significant figures and the three lower dots represent voltage, 
tolerance, and multiplier reading, respectively, from left to right. 
Colors are as follows : 



Color 


Significant 
Figure 


Voltage 


,o,e ra „ce 


Multiplier 


black 


0^ 






1 


brown 


1 


100 


1% 


10 


red 


2 


200 


2% 


100 


orange 


3 


300 


3% 


1,000 


yellow 


4 


400 


4% 


10,000 


green 


5 


500 


5% - 


100,000 


blue 


6 


600 


6% 


1,000,000 


violet 


7 


700 


7% 


10,000,000 


gray *~ 


8 


800 


8% 


100,000,000 


white 


9 


900 


9% 


1,000,000,000 


gold 


— 


1000 





.1 


silver 


— 


— 


. io% 


.01 


no color 


~ 


500 


20% 


— 



143 



INDEX 

Paragraphs Page 

Abbreviations Ann T 141 

AP amplifiers . - ^ZZZZZiooTloSS??! ATM. 

Alignment:- 118 ' 141 63 ' 78 

Receiver . 93,97,98, 41,44,45, 

Transmitter lSs'lli Bl'S 

Ammeters £^59 5 J'|f 

Amplitude modulation IZZHZL7I 109 54 

AStomS'f^^^Trat^rZT 1 60,153,182 23,82,94 

BatSs? V ° IUme C ° ntr01 " 81 ' 124 30 ' 64 



Dry cell ,.-, 

Storage ZZZZZT " 



23 
62 24 



Beat frequency oscillator 101 AQ 

Bonding |$ 4f 

Bridging i«i?S ol 

Broad tuning raetoTIIZ: 'l rq f! 

Buffer amplifier ... Jn* 2? 

Bypass 1==ZZ=IZ=Z 79,19? 30,10u 

Calibration 



Ammeters ... 
Receivers ... 



9 5 

92 40 



Transmitters -.^ 2J{ 

Voltmeters 1 1 °| 5 ° 

Circuit disturbance ._ 109 J% 

Circuit breakers ZZZ ^ Zo 



Circuit, power supply 
Dynamotor 
Generator 



54 22 

86 33 



Nonsynchronous vibrator 07 q? 

Rectifier**:.. " Ik tt 

Synchronous vibrator qq q2 

Circuits, special: """"" oy d ' 

Automatic volume control qi qa 

Bypass _._ „ '"'"- Z* j™ 

Delayed automatic volume control""™ 82 32 

Manual volume c^rofliZZZZZ^ZZZ 80 |q 

145 



Paragraphs Page 

Circuits, vacuum tube: 

Control grid 79 17Q 00 00 

™? t r nt — ~~ - ~ %i?l I;l 

sS£»ia«==z=r -" 4 S 27 f 8 

Suppressor grid 74 or 

Cleaning 1153,164 82, 8 ! 

Coils 46 14g 17 ' 

Condensers 44,149,App. II 15,81 142 

Connections, electrical 48,145 21 80 

Cords _ ^0*0-1 

OSSSJ? IndiCa *^" :: === = 28,32 10,12 

SuK^' 6 reCtifiGr :== - 5 % 8 fi 8 23,35 

Crystals ZZZZZZZZZZ 56 22 

DC amplifiers 102 4g 

Dead receiver _ 16 4 q2 

Decoupling . L 7g °' 

Delayed automatic volume control 82 32 

Detector, diode 98 119 45 63 

Discriminator 99(125 46 6 J 

Distorted reception 165 ' 8 ~ 

Distorted transmission .. " 186 qk 

Dynamotors " "~qa g fi -i c 4 9A 00 g. 

Electron-ray voltmeter ___ZZZZZZZZ 23 * ' 9 
Extending range: 

Ammeters ja r 

Ohmmeters Z~ 29 11 

Voltmeters ZZZZZZZ 19 7 

Fading receiver . 171 qq 

False zero ZZZ 22 7 

Xf 3 T 1— T J ~ZZZ77,157,196 30,83,100 

Final operating check : . 

Receivers . j 84 04 

Transmitters 188 «;: 

Frequency meter 35 ^ 

Frequency modulation ... . iin kk 

Full-wave rectifier . or ,? 

Fuzes zzzzzzzzzz: 53,151 22J2 

Generators 63,84,154 24,32,83 

Grid modulation jq 9 ' 54 

Half-wave rectifier oo op 

Headphones: 35 

Re s pia7emen-r::;;:::;; 25 - 30 £l 10 > n >g 

Troubles ZZZZZZZZZZ 66.175 25,9? 

S? ;*• 167 8 7 

lr amplifiers 9 g j20 43 64 

Incandescent lamp ZZZZZZl 5,24,31 6,9,'ll 

146 



Paragraphs Page 

Inspection ; —160,162,165 85,86,87 

Intermittent receiver 172 gg 

Internal resistance: 

Ammeter g 4 

Voltmeter :.. _""" 1 7 q 

Jacks . ;_ 50 21 

Lead wires _' __ 146 80 

Limiter — 97 44 

Loudspeaker . "(57,156,175 25,83,91 

Maintenance, echelons of . .._.__ 2 1 

Manual volume control ■ " 80 30 

Motors, general ~"\. 59 23 

Microphones „. 68 25 

Mixer "" 94 42 

Modulation : 

AM ._ . 109 54 

b M 11Q 55 

Multiplier, frequency 106 53 

Neon lamp indicators 14 g 

Neutralization . ^ "___"___ 105 '51 

Noise, vehicular 190-193 99 

Noise, suppression _-_ " " 194-199 100 

Noisy receiver 166 87 

Non-lmear coil HO 55 

Ohmmeter 29 11 

Operator, radio 2 1 

Oscillating receiver .... _"; 92,168 40,88 

Oscillator: 

Beat frequency ... 101 48 

RF, receiver : 93>1 21 41,64 

Signal generator 34 12 

Transmitter """.104,138 50,76 

Output meter 27 10 

Plate modulation 109 54 

piugs ;:__:;;;;;:;; 5 o 21 

Power, measurement of 26-27 10 

Power, amplifier: 

Receiver 100 47 

Transmitter __„ 107 53 

Power output 170 gg 

Power supply ~~Z!"r83-90,116, 32,61, 

o i, 11 ,.*■ 137,174 76,90 

Push pull amplifiers :. 107,117 53,63 

Push push amplifiers 106 ^ 

Radio Set SCR-284 : " * 

BFO alignment, receiver ._ 237 133 

Buffer amplifier alignment, transmitter 240 134 

Common troubles „. .244 137 

Final operating check LZ!!!™"ZZ~ 245 138 

147 



129 
119 
120 



tit ,. . . Paragraphs Page 

IF alignment, receiver 2 q 7 ,«„ 

Inspection ?4 o 7,.* }** 

Neutralization.......;.".;;;;;;;;;;;;;""; - 24? ]* 4 . 

Oscillator alignment, transmitter ... 239 iql 

Power amplifier alignment, transmitter " 240 is? 

RF alignment, receiver " """ 2 38 ioS 

Radio Set SCR-608: " " " 6 

Common troubles . 232 197 

Discriminator alignment, receiver 220 i9n 

Doubler alignment, transmitter ...... 228 i9q 

Final operating check . " 233 

IF alignment, receiver """"l"'" 219 

IF oscillator alignment, receiver 221 

Inspection 230-231 19s 

Oscillator alignment, transmitter 225 191 

Power amplifier alignment, transmitter 220 19q 

Presetting channels 214-217 11 « 

Rectifier alignment, transmitter """"" 226 199 

RF alignment, receiver . 999 \%i 

Test set ..;.... . ~ ~" %\\ }|0 

Tnpler alignment, transmitter . 997 J™ 

Radio Set SCR-610: """ ' 122 

Common troubles 211 119 

Discriminator alignment, receiver ""; 206-207 ins 

Final operating check " "" " |l 2 11! 

ImJST** reCdVer —==^™ 205,207 107408 

KaStion"-;;;;;;; - — 209 1Jg }£ 

Presetting channels: * 109 

Field method 90 o 1A „ 

Precise method 909 |2f 

Preparation for . . "ZZ " 20? J2S 

Reactance modulator " "" " TYa *t 

Receivers: LW 55 

lystems " 91 " 102 39 

RecSie?s: 113 ~ 133 ™ 

Copper oxide c 7 8S g „ „_ 

Full-wave 57 'ff 23 *j5 

Full-wave bridge ZZ 00 3 5 

Half-wave ....... ~ *| 35 

Relays "" 

RepairmanV^radio" ZZZZ" 2T1 4 H «? 1 o ™'2 2 

£S3£™- — ==r=r=^}g;JS 1A gg 

»£Sirii-s*st=rz— 43 ' 147 '^i 15 - 8 ] b 14 ei 

I? Sj SSS* - :: =«f f 61 |; 

Selectivity ZZ" ~~i"~ ^ 



148 



144 79 

86 



c , . , ,. Paragraphs Page 

stnltTourcerzz;;:; «w# *uo$ 

Signal tracers "Z " — og^T 12 >]» 

Signal tracing 197 -.on ki 

Stage muting ZZZZZZ 133 ?5 

Superheterodyne receivers ._ oi 11 c 00 cf 

Suppressors __ J1 '|Jf ^J 

Switches "ZZZZZ 51 21 

Test equipment o_ 5 „ 

Thermocouples c Q O o 

Tracking ~ Q o .25 ,, £* 

Transformers JZZZZI UfA frg 

Transmitter systems, testing ZZZ 134-141 jl 

Troubles, receiver, common _ 'l 70 ™ nX. 

Troubles, transmitter ... is? aa 

tt V 7 ■ ; — 47,150,176 19,82,91 

Untuned signal tracer . . ao \a 

Vacuum tube: 14 

Stages, receiver, testing . 114 fin 

Stages, transmitter, testing . "" "is4_i4i 7* 

Voltmeter to 

(See also Tubes) 



22 7 

vffiS interferenCe - 189-199 98 

vibrators . 65,87,89,155 24,34, 

Voltmeters , fi9 ,, Q 3 J'^ 

Volume control ZZZZZZZZiZZZ 16 ^ *H 

Weak receiver 170 gg 



FAS, Fort Sill, Okla., (4-2744—3,500)— 29197 



149